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+The Project Gutenberg EBook of The Postnatal Development of Two Broods of
+Great Horned Owls, by Donald F. Hoffmeister and Henry W. Setzer
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: The Postnatal Development of Two Broods of Great Horned Owls
+       Bubo virginianus
+
+Author: Donald F. Hoffmeister
+        Henry W. Setzer
+
+Release Date: January 31, 2011 [EBook #35118]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE POSTNATAL DEVELOPMENT OF ***
+
+
+
+
+Produced by Chris Curnow, Joseph Cooper, Josephine Paolucci
+and the Online Distributed Proofreading Team at
+http://www.pgdp.net.
+
+
+
+
+
+
+
+THE POSTNATAL DEVELOPMENT OF TWO BROODS OF GREAT HORNED OWLS
+
+(Bubo virginianus)
+
+BY
+
+DONALD F. HOFFMEISTER AND HENRY W. SETZER
+
+University of Kansas Publications
+
+Museum of Natural History
+
+Volume 1, No. 8, pp. 157-173
+October 6, 1947
+
+UNIVERSITY OF KANSAS
+LAWRENCE
+1947
+
+
+UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY
+Editors: E. Raymond Hall, Chairman, Donald S. Farner, H. H. Lane,
+Edward H. Taylor
+
+Volume 1, No. 8, pp. 157-173
+October 6, 1947
+
+UNIVERSITY OF KANSAS
+Lawrence, Kansas
+
+PRINTED BY
+FERD VOILAND, JR., STATE PRINTER
+TOPEKA, KANSAS
+1947
+
+21-6958
+
+
+
+
+The Postnatal Development of Two Broods of Great Horned Owls
+
+(_Bubo virginianus_)
+
+By
+
+DONALD F. HOFFMEISTER AND HENRY W. SETZER
+
+
+Opportunity regularly to observe at the nest the development of young
+Great Horned Owls, _Bubo virginianus_ (Gmelin), under favorable
+conditions, was afforded when a pair nested and reared their three
+offspring in 1945 and one offspring in 1946 on the vine-covered north
+wall of the Museum of Natural History at the University of Kansas. The
+observations here reported are based primarily on the three young raised
+in 1945 when daily observations were made. These have been supplemented
+by other observations made of the one nestling in 1946. Unless otherwise
+stated, observations pertain to the nest and three young in 1945.
+
+
+NEST SITE
+
+In 1945 the nest was situated on a metal-covered cement ledge, two feet
+wide and 48 feet above the ground, at the northeast corner of the Museum
+Building. The nest was protected on the east by a stone abutment of the
+building and on the south by the north wall of the building itself. Here
+the nest could be observed at will through a laboratory window without
+disturbing the birds. The taking of notes was begun at the time of
+egg-laying and extended to the time at which the young left the nest,
+February 3 through April 26, 1945. In 1946 the owls nested farther down
+the north side of the building, behind two cement pillars, approximately
+25 feet above the ground. To examine the nest in 1946 it was necessary
+to lower an observer down the side of the building by means of a rope.
+Observations of this nest were never made more frequently than every
+other day. The adult owls were first seen at the nest on February 3,
+1946; careful examination of the nest began when the one egg hatched on
+March 7 and continued until April 25, shortly before the young owl left
+the nest.
+
+One large cottonwood tree, used by the parent-owls as a landing place
+whenever they were forced from the nest, was situated approximately 110
+feet to the north and a five-story building was located 80 feet farther
+to the north. Numerous smaller trees line the street to the east and
+there are some on the lawns around the Museum. Also, there are about two
+acres of trees 225 feet west of the nest-site where the parent-owls took
+refuge when forced from the cottonwood tree.
+
+The nest, if it can be called a nest, was no more than a few bare
+branches of the Virginia creeper, which covers the side of the building,
+together with some excrement which the owls tended to push to the
+periphery of the nest. For most of the time the three eggs in 1945 lay
+directly on the metal which covered the ledge, because there was no
+definite floor to the nest. The single egg in 1946 lay on the cement
+shelf between the pillars and the wall of the building. This laxity in
+nest building by Great Horned Owls apparently is not uncommon (see Bent,
+1938:300).
+
+
+PERIOD OF INCUBATION
+
+Incubation of the eggs probably began, in 1945, on February 5, the day
+the first egg was laid. It has usually been assumed that, in birds of
+prey, incubation begins when the first egg is laid. The last of the
+three eggs was laid February 7. In 1946, the single egg was being
+incubated on February 4. Since another egg had been laid two or three
+days before this--a broken egg was found beneath the nest and there were
+remnants of the egg in the nest--incubation may have started as early as
+February 1 or February 2. In comparing these dates of initial incubation
+with other recorded dates of nesting, only those from places at, or
+near, the latitude of Lawrence, Kansas, in the central United States,
+should be expected to be approximately the same since the times of
+egg-laying and incubation are progressively later in the year as
+approach is made toward the polar region. Baumgartner (1938:279) has
+previously pointed this out.
+
+The incubation period for the Great Horned Owl in the central United
+States has usually been regarded as 28 to 29 days. In the nest under
+observation in 1945, two eggs hatched on March 12 and are assumed to be
+the first two eggs laid, with an incubation period for each of 35 and 34
+days, respectively, and the third egg hatched on March 14 with an
+incubation period of 35 days. In 1946, the single egg hatched on the
+33rd day, assuming that incubation began on February 2, for the egg
+hatched March 7. In the period of egg-laying and also in incubation, the
+parent bird in 1945 was frequently disturbed by persons who peered at it
+through the window. Curious observers handled the eggs at least once and
+vigorous pounding by carpenters in the room adjacent to the nest
+frequently flushed the adult bird but did not cause desertion of the
+nest. It may be that such disturbances prolonged the incubation period.
+However, in 1946, the brooding birds were undisturbed, yet the
+incubation period was nearly as long. If an observer near the nest
+exposed himself in the daytime to the incubating bird, the adult flew,
+but exposure at 50 feet or more from the nest only caused the incubating
+bird to remain alert on the nest. When flushed, the parent usually
+returned to the nest within 15 minutes or less after the observer
+withdrew. On the thirty-second and thirty-third days of incubation in
+1945, the crew of carpenters demolished partitions within the building
+on which the owl was nesting, and within 15 feet of the nest itself. At
+first the adult would fly from the nest at each outburst of hammering
+and, at one time, remained away from the nest for more than two hours.
+After a few hours of intermittent hammering, however, the parent bird
+remained on the nest despite all the noise produced. These observations
+bear out, rather than refute, Baumgartner's statement (1938:281) that
+"the horned owl incubates very closely," for a strong stimulus was
+necessary to keep the owl from covering the eggs.
+
+The egg hatched on March 14, 1945, and approximately two days later than
+the other two, is judged to be the one laid last. This owl, III, was
+always 5 to 21 per cent lighter in weight than the older birds when
+weights for corresponding ages were compared. Whether this difference
+was the result of a lack of food because of dominance of the two older
+birds, or because of a sexual difference, we do not know. The owl that
+hatched in 1946 was likewise markedly lighter than the first two birds
+hatched in 1945 (figure 1). A series of adults from Meade County,
+southwestern Kansas, shows a pronounced secondary sexual difference in
+weight. In this sample the mean weight of 17 males, 1,208 grams, was 21
+per cent less than that of 25 females, 1,531 grams.
+
+
+GROWTH OF JUVENILES
+
+The principal measurement of growth taken by us was the weight of the
+owls. In 1945 each of the three owls was weighed daily, with two or
+three exceptions when a 48-hour period was interposed between weighings.
+The young were removed from the nest to a nearby balance, weighed, and
+examined. The owl last hatched (owl III) was weighed on the first day of
+life and on most subsequent days. The other two owls (designated as owls
+I and II) were first weighed when they were between 53 and 60 hours
+old. On some days the birds were weighed twice, once in the morning and
+once in the late afternoon; on most days, they were weighed only in the
+late afternoon. The owl hatched in 1946 was weighed when seven days old
+and at irregular, but usually two day, intervals thereafter. It was
+weighed always slightly before midday.
+
+[Illustration: FIG. 1. Growth of four Great Horned Owls as shown by
+changes in weight from near the time of hatching until the time of
+leaving the nest.]
+
+The growth of the four owls is well shown by the changes in weight
+recorded in figure 1. For the period during which the young owls
+remained at the nest, growth can be divided into two phases: (1) a
+rapid increase in weight during the first 3-1/2 or 4 weeks while the
+parent birds are supplying the young with ample food; and (2) a
+subsequent period of slower growth, marked by fluctuations (actual
+losses as well as gains) in weight resulting from the failure of the
+parent birds to provide an ample supply of food. If there is an initial
+period of about one week in postnatal development in which there is a
+rather slow gain in weight, as suggested by Sumner (1933:284), it was
+poorly marked in this instance. Owl IV remained at the nest until the
+50th day of age, and on the 47th and 49th days (not shown on chart, fig.
+1) weighed 1,011.4 grams and 971.4 grams, respectively. By this age, the
+growth curve had definitely flattened out. The fact that owl IV was
+consistently heavier than owl III might be accounted for, in part, by
+the fact that owl IV was always weighed in the morning when it was
+gorged with food. However, Riddle, Charles, and Cauthen (1932) have
+pointed out that when there were two or more pigeons in a nest, each
+grew less rapidly than if there was only one present.
+
+Within about 12 hours after hatching, the smallest of the three owlets
+(III) weighed 48.7 grams. During the first four weeks of postnatal
+growth, each owl gained in weight, daily, an average of 33-1/3 grams or
+an increase of 11.1 per cent. Owl I gained an average of 36.1 grams each
+day, or a daily increase of 10.7 per cent; owl II, 37.8 grams, or 11.2
+per cent; and owl III, 26.1 grams, or 11.4 per cent. From the beginning
+of the fifth week until the time the young left the nest, the three owls
+gained on the average only 12.7 grams or approximately 1.6 per cent in
+weight daily. Individually, the daily mean increases were as follows: I,
+9.6 grams or 0.93 per cent; II, 12.7 grams or 1.86 per cent; III, 15.8
+grams or 1.97 per cent. Prior to the twenty-sixth to twenty-eighth day
+of age, there seldom was any loss in weight from day to day, whereas
+after this period, approximately one weight in four was less than on the
+previous day. These data support the earlier statement that during the
+first 3-1/2 or 4 weeks, there is a relatively uniform and rapid increase
+in weight whereas after this period weight fluctuates.
+
+Growth as measured by changes in weight in these young Great Horned Owls
+parallels growth in some other young birds. For example, nestling
+Red-tailed Hawks, as reported upon by Fitch, Swenson, and Tillotson
+(1946:215), increase in weight rapidly for about the first three weeks
+and then more gradually. Sumner's (1929b) graphs indicate the same
+pattern of growth in the Barn and Great Horned owls and Red-tailed and
+Cooper hawks. Pigeons, judging from the growth curves for bodily weight
+as given by Riddle, Charles, and Cauthen (1932), increase in weight
+rapidly until somewhere between the twenty-fourth and thirty-second day
+of postnatal development. However, in the Golden Eagle, the early part
+of postnatal development is not one of rapid growth, judging from
+Sumner's diagram (1929a:164), but after the fourth week there is a rapid
+increase in weight. Graphs that Sumner (1929b) gives for Sparrow Hawks,
+Long-eared Owls, and Screech Owls, indicate that in these instances also
+the increase in weight during the first few days of postnatal
+development was not so rapid as it was after the end of the first week.
+Stoner (1935) indicates that in the young Barn Swallow, increase in
+weight was most rapid between the fourth and tenth days, with the young
+remaining at the nest until the twentieth day. Much the same pattern of
+weight increase was noted by Stoner (1945) in the Cliff Swallow.
+Huggins' (1940:228) sigmoid curve for increase in weight in the House
+Wren indicates that the period of rapid growth in this species does not
+begin until the second day. Sumner (1934:284) cites other studies which
+he believes, for altricial birds, indicate three periods of growth, an
+initial period of rather slow gain, a period of maximum increase in
+weight, and a final period of fluctuations. As previously indicated, for
+the Great Horned Owls under observation, and in some other species as
+indicated by published growth curves, the initial period of slow gain is
+lacking.
+
+The period of a decelerated rate of growth in the young Great Horned
+Owls is correlated with the occasional lack of food. The parent birds,
+during this latter period, remain off the nest more of the time during
+the day, and their failure to provide the young with food may represent
+an attempt to force the young to become proficient in flight or to force
+them away from the nest site, which amounts to the same thing. When only
+slightly more than a month old, the young began to test their wings,
+springing into the air, and, in general, becoming more active and alert.
+Sumner (1929b:110) has suggested some other possible reasons for the
+period of decelerated rate of growth.
+
+Although there was a daily increase in weight in the early stages of
+growth, there was a decided fluctuation in any twenty-four hour period.
+On any given day, the young always were heavier in the morning than in
+the afternoon (see figure 2); presumably they were gorged with food
+early in the morning.
+
+[Illustration: FIG. 2. Morning and afternoon weights of two Great Horned
+Owls. Note that in the morning the owls weigh more than in the
+afternoon.]
+
+When the young left the nest, they were approximately three-fourths
+grown. When owl I on the 44th day and owl II on the 45th day left the
+nest, they weighed 1,120 and 1,139 grams, respectively, or 73 and 74 per
+cent, respectively, of the average weight of 25 females (1,530.9 grams).
+Owl III weighed 943.3 grams on the 43rd day and owl IV weighed 971.4
+grams on the 49th day, or 78 and 80 per cent, respectively, of the
+average weight of 17 mature males (1,207.7 grams). Owl I left the nest
+18 hours before owl II did. Owl III attempted to leave when 43 days old,
+but for it coördinated flight was impossible and the bird landed on the
+lawn after a 150-foot glide. When attempting to take owl IV from the
+nest on the 49th day, it sprang into the air and by gliding, aided by an
+occasional flap, sailed more than 300 feet before alighting on the
+ground. After we returned the owl to the nest, it immediately sailed
+forth for even a longer distance. When attempt was made to pick up owls
+III and IV after they had flown down to the ground, they rolled over on
+their backs and used both claws and beaks defensively. Such a reaction
+never was noted at the nest; there our hands were inspected, and
+sometimes bitten by the owls as possible sources of food, but the claws
+were rarely used offensively or defensively.
+
+Slightly elevated remiges and rectrices, still in the sheath, were
+visible on the ninth day. Some remiges first ruptured the feather sheath
+on the 14th day; nearly all of the primaries ruptured the sheaths by the
+19th day and the secondaries by the 20th day. The amount of eruption
+from the sheath for primaries, secondaries, and rectrices, as given in
+table 1, was determined by measuring the one feather of, say, the
+secondaries, judged to be near the mean in degree of eruption. The
+feathers of the wing at 21 and 47 days of age are shown in figure 5. On
+the eighth day, or slightly before, the white nestling down of the newly
+hatched bird was replaced by a downy immature plumage, which was more
+yellowish than the preceding plumage. The development of the plumage in
+the birds under observation was much the same as that recorded by Sumner
+(1933) in _Bubo virginianus pacificus_ Cassin.
+
+
+TABLE 1.--Changes with age in certain parts of a young Great Horned Owl
+hatched in 1946.
+
+(Measurements are in millimeters)
+
+==========================+======+======+======+======+======+======+======
+Age in days               |  19  |  21  |  26  |  33  |  37  |  39  |  49
+--------------------------+------+------+------+------+------+------+------
+Length of erupted portion |      |      |      |      |      |      |
+of "average" primary      |  6.0 | 10.0 | 26.0 | 93.0 | 87.5 | 99.2 |......
+--------------------------+------+------+------+------+------+------+------
+Length of erupted portion |      |      |      |      |      |      |
+of "average" secondary    |......|  5.0 | 25.0 | 60.0 | 78.0 | 95.0 |......
+--------------------------+------+------+------+------+------+------+------
+Length of erupted portion |      |      |      |      |      |      |
+of "average" rectrix      | 17.0 |......| 16.0 | 28.0 |......|......| 78.0
+--------------------------+------+------+------+------+------+------+------
+Length of exposed culmen  |      |      |      |      |      |      |
+without cere              |......| 19.6 |......| 22.5 |......| 24.0 | 23.7
+--------------------------+------+------+------+------+------+------+------
+Length of total culmen    |......| 30.4 |......| 36.0 |......| 38.5 | 40.0
+--------------------------+------+------+------+------+------+------+------
+Length of femur           | 69.0 |......|......| 87.5 |......| 89.5 |......
+--------------------------+------+------+------+------+------+------+------
+
+Traces of the egg-tooth were retained until the ninth day in two owls
+and until the 11th day in another. In owl IV, the egg-tooth was lost
+sometime between the 9th and 14th day. Changes in the length of the
+culmen are indicated in Table 1. The length of total culmen of owl IV
+when 47 days old is slightly greater than the average for three adult
+male owls from Lawrence, Kansas (40.0 mm. as contrasted with 39.2 mm. in
+the adults). The length of exposed culmen, without cere, in the same
+bird when 47 days old is less than the average of this measurement in
+the three adults from Lawrence (23.7 mm. as contrasted with 26.5 mm. in
+the adults). The femur was measured on three occasions as accurately as
+possible through the skin and flesh. The precise boundaries of the
+femur could not be determined and the thickness of the skin and certain
+muscle is included. These measurements are not given to indicate actual
+length of the femur, but to indicate the relative changes in length of
+this bone.
+
+Remnants of the yolk stalk were clearly evident at seven days of age
+(see fig. 5) in the owl hatched in 1946 and were still present when the
+owl was last examined (49 days of age) just before the young left the
+nest. The scablike remnant was not noted in the three young hatched in
+1945, but close inspection was not made to see if it was present.
+
+The instinctive reactions of young horned owls shortly after hatching
+have been fully described by Sumner (1934). By the third day our owls
+could raise their heads, but when a bird was undisturbed its head lay on
+the nest floor and the wings were slightly spread. The eyes of owls I
+and II opened at about 7-1/2 days, those of owl III on the 6th day, and
+those of owl IV sometime between the 7th and 9th days. After the eyes
+opened, the head was held erect more of the time. The young responded
+with "cries" when disturbed by handling, when stimulated by certain
+movements of the parent, or by movements of our hands near their heads,
+which suggested to them the possibility of being fed. Cries were evident
+but weak in the unhatched, pipped, egg, but soon after hatching
+increased in intensity, and beginning at six days of age were replaced,
+in part, by the characteristic "bill-snapping" of more mature birds.
+These cries of the young may serve, among other things, for recognition,
+inasmuch as they were given when the parent was inspecting the young.
+When the parent returned to the nest and covered the young, after having
+been flushed, it sometimes uttered a special note, "hut, hut, hut," much
+like the "cluck" note of the hen of the domestic chicken. The young
+responded to these notes with faint cries, in contrast to the loud cries
+signifying alarm and possibly hunger, which they gave when the parents
+were absent from the nest.
+
+The first definite evidence that the young were attempting to feed
+themselves was obtained when they were 23 days old. Frequently
+thereafter, fresh blood was found on their beaks and claws, but as late
+as the 34th day a parent was seen feeding them. That day, after being
+flushed, a parent returned to the nest at 7 p. m., and began tearing
+away parts of a cottontail which had previously been brought to the
+nest. Bones in the hind leg of the rabbit broke readily under pressure
+of the parent's bill, and the three young crowded in close, opening
+their bills widely and placing them around that of the parent. Of
+cottontails, the only parts consistently uneaten were the upper cheek
+teeth and the supporting maxillae and connecting palatal bridge.
+
+
+FOOD BROUGHT TO THE NEST
+
+In the 45 days that the young remained at the nest site in 1945,
+ninety-one individuals of 16 different species of animals were brought
+by the parent owls (table 2). Probably a few smaller animals, of which
+we saw no traces, were caught and eaten at night. In 1946, two
+additional kinds of birds were brought to the nest: 1 Baldpate (_Mareca
+americana_) and 1 Pied-billed Grebe (_Podilymbus podiceps_). The large
+number of Rock Doves in the list can be explained by their abundance on
+the buildings on the University campus, including the Museum building
+where some were nesting as close as 100 feet to the owl nest. When the
+owls were less than a week old, only small birds and mammals were
+brought (young Rock Doves, Robins, Starlings, Grasshopper Sparrow,
+meadow mouse, and Norway rat). The first rabbit was brought when the
+owls were eight days old.
+
+
+TABLE 2. Number of food items brought to the nest by the Great Horned
+Owls in 1945
+
+    Birds
+
+    Rock Dove (_Columba livia_)                       32
+    Robin (_Turdus migratorius_)                       6
+    Starling (_Sturnus vulgaris_)                      4
+    Mourning Dove (_Zenaidura macroura_)              10
+    Meadowlark (_Sturnella sp._)                       3
+    Red-wing (_Agelaius phoeniceus_)                   1
+    Bronzed Grackle (_Quiscalus versicolor_)           1
+    Mockingbird (_Mimus polyglottos_)                  1
+    Brown Thrasher (_Toxostoma rufum_)                 1
+    Grasshopper Sparrow (_Ammodramus savannarum_)      1
+    Coot (_Fulica americana_)                          3
+    Sora (_Porzana carolina_)                          1
+    Blue-winged Teal (_Anas discors_)                  1
+
+    Mammals
+
+    _Sylvilagus floridanus_                           19
+    _Rattus norvegicus_                                6
+    _Microtus ochrogaster_                             1
+
+After the 28th day, only 18 food items, or slightly less than 20 per
+cent of the total number, were brought to the nest. These last 18 food
+items brought after the owls were 4 weeks old were no larger or bulkier
+than those brought in the previous 20 days. The beginning of this period
+of reduced amount of food corresponds to the beginning of the second
+phase of growth characterized by marked fluctuations in weight.
+
+Fox squirrels (_Sciurus niger_) are abundant on the University campus,
+yet there were no remains of this mammal at the nest. This may be
+explained by the fact that fox squirrels are principally diurnal and
+Great Horned Owls feed principally at night. Yet in early February,
+1946, when the owls were preparing the nest, they frequently flew on and
+off the nest in the early twilight of evening while one or two fox
+squirrels fed in the periphery of trees not more than 25 feet away. Yet
+the owls flew off to the west and left this source of food unmolested.
+
+Whether both owls regularly attended the young we do not know, for the
+adults were not distinctively marked. On March 17, 1945, when weighing
+the young, one parent bird started to return to the nest but was
+frightened away by the observer who at the same time noted the other
+parent perched in an adjacent tree. This was the first time two adults
+were seen at the same time near the nest. In 1946, two adult owls
+(presumably both were parents) were within sight at one time when the
+young owl first sailed forth and landed in a wooded area some 100 yards
+away.
+
+
+SUMMARY
+
+Great Horned Owls (_Bubo virginianus virginianus_) have employed as nest
+sites the protruding shelves of the stone wall of the Museum of Natural
+History at the University of Kansas for several years. In 1945, daily
+observations were made on one such nest and its three young, and in 1946
+irregular observations were made on another such nest and the one young
+owl. The incubation time for the three owls, hatched in 1945, was 35
+days for two of the young and 34 days for the third; for the one owl
+hatched in 1946, the incubation time was at least 33 days. Two owls were
+consistently smaller; when these smaller two left the nest they were,
+respectively, 21 and 17 per cent lighter than the other two. The smaller
+two were judged to be males because adult males in Kansas average
+smaller by 21 per cent than adult females.
+
+Growth of the nestling young is divisible into (1) a period of rapid
+increase in weight during the first 25 to 28 days, and (2) a subsequent
+period marked by gains and losses in weight. The fluctuations in this
+latter period are correlated with a reduction in food brought to the
+nest by the parent birds and with the development of habits of flight.
+This second period may be considered to be a period of "weaning." By
+the forty-fifth day, the young owls are able to fly short distances and
+thus are able to leave the site of the nest permanently. At this time
+they are about three-fourths grown.
+
+Ninety-one individuals of 16 species of birds and mammals made up the
+food items brought to the nest in 1945. Two factors seem to be concerned
+in the acquisition of prey: (1) its availability and (2) appropriate
+size of the prey.
+
+
+
+
+LITERATURE CITED
+
+BAUMGARTNER, F. M.
+
+1938. Courtship and nesting of the Great Horned Owls. Wilson Bull.,
+50:274-285.
+
+
+BENT, A. C.
+
+1938. Life histories of North American birds of prey (Part 2), Orders
+Falconiformes and Strigiformes. U. S. Nat. Mus., Bull. 170, viii + 482
+pp.
+
+
+FITCH, H. S., SWENSON, F., and TILLOTSON, D. F.
+
+1946. Behavior and food habits of the Red-tailed Hawk. Condor,
+48:205-237.
+
+
+HUGGINS, S. E.
+
+1940. Relative growth in the House Wren. Growth, 4:225-236.
+
+
+RIDDLE, O., CHARLES, D. R., and CAUTHEN, G. E.
+
+1932. Relative growth rates in large and small races of pigeons. Proc.
+Soc. Exp. Biol. and Med., 29:1216-1220.
+
+
+STONER, D.
+
+1935. Temperature and growth studies on the Barn Swallow. Auk,
+52:400-407.
+
+1945. Temperature and growth studies of the Northern Cliff Swallow. Auk,
+62:207-216.
+
+
+SUMNER, E. L., JR.
+
+1929a. Notes on the growth and behavior of young Golden Eagles. Auk,
+46:161-169.
+
+1929b. Comparative studies in the growth of young raptores. Condor,
+31:85-111.
+
+1933. The growth of some young raptorial birds. Univ. California Publ.
+Zoöl., 40:277-307.
+
+1934. The behavior of some young raptorial birds. Univ. California Publ.
+Zoöl., 40:331-361.
+
+
+[Illustration: FIG. 3. Young Great Horned Owls in nest. Two owls are 7,
+12, 18, 32, and 36 days of age, respectively; the third owl is about 2
+days younger in each instance.]
+
+[Illustration: FIG. 4. Young Great Horned Owl hatched in 1946. The two
+lower pictures show the developing facial mask. Photographs by Joćo
+Moojen.]
+
+[Illustration: FIG. 5. Young Great Horned Owl hatched in 1946. Upper
+row: Ventral views showing scar of yolk sac and ventral side of wing.
+Middle row: Ventral (left) and dorsal view of wing at 21 days. Bottom
+row: Ventral (left) and dorsal view of wing at 47 days. Photographs by
+Joćo Moojen.]
+
+
+21-6958
+
+
+
+
+
+End of the Project Gutenberg EBook of The Postnatal Development of Two
+Broods of Great Horned Owls, by Donald F. Hoffmeister and Henry W. Setzer
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+    <meta http-equiv="Content-Type" content="text/html;charset=iso-8859-1" />
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+      The Project Gutenberg eBook of The Postnatal Development Of Two Broods Of Great Horned Owls, by Donald F. Hoffmeister And Henry W. Setzer.
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+<pre>
+
+The Project Gutenberg EBook of The Postnatal Development of Two Broods of
+Great Horned Owls, by Donald F. Hoffmeister and Henry W. Setzer
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: The Postnatal Development of Two Broods of Great Horned Owls
+       Bubo virginianus
+
+Author: Donald F. Hoffmeister
+        Henry W. Setzer
+
+Release Date: January 31, 2011 [EBook #35118]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE POSTNATAL DEVELOPMENT OF ***
+
+
+
+
+Produced by Chris Curnow, Joseph Cooper, Josephine Paolucci
+and the Online Distributed Proofreading Team at
+http://www.pgdp.net.
+
+
+
+
+
+
+</pre>
+
+
+
+<h1>THE POSTNATAL DEVELOPMENT OF TWO BROODS OF GREAT HORNED OWLS</h1>
+
+<h3>(Bubo virginianus)</h3>
+
+<h3>BY</h3>
+
+<h2>DONALD F. HOFFMEISTER AND HENRY W. SETZER</h2>
+
+<p class="center">University of Kansas Publications</p>
+
+<p class="center">Museum of Natural History</p>
+
+<p class="center">Volume 1, No. 8, pp. 157-173</p>
+
+<p class="center">October 6, 1947</p>
+
+<p class="center">
+UNIVERSITY OF KANSAS<br />
+LAWRENCE<br />
+1947<br />
+<br />
+<br />
+<span class="smcap">University of Kansas Publications, Museum of Natural History</span><br />
+<br />
+Editors: E. Raymond Hall, Chairman, Donald S. Farner, H. H. Lane,<br />
+Edward H. Taylor<br />
+<br />
+Volume 1, No. 8, pp. 157-173<br />
+October 6, 1947<br />
+<br />
+<span class="smcap">University of Kansas</span><br />
+Lawrence, Kansas<br />
+<br />
+PRINTED BY<br />
+FERD VOILAND, JR., STATE PRINTER<br />
+TOPEKA, KANSAS<br />
+1947<br />
+<br />
+21-6958<br />
+</p>
+
+
+
+<hr style="width: 65%;" /><p><span class='pagenum'><a name="Page_159" id="Page_159">[Pg 159]</a></span></p>
+<h2>The Postnatal Development of Two Broods of Great Horned Owls</h2>
+
+<h3>(<i>Bubo virginianus</i>)</h3>
+
+<h4>By</h4>
+
+<h3>DONALD F. HOFFMEISTER AND HENRY W. SETZER</h3>
+
+
+<p>Opportunity regularly to observe at the nest the development of young
+Great Horned Owls, <i>Bubo virginianus</i> (Gmelin), under favorable
+conditions, was afforded when a pair nested and reared their three
+offspring in 1945 and one offspring in 1946 on the vine-covered north
+wall of the Museum of Natural History at the University of Kansas. The
+observations here reported are based primarily on the three young raised
+in 1945 when daily observations were made. These have been supplemented
+by other observations made of the one nestling in 1946. Unless otherwise
+stated, observations pertain to the nest and three young in 1945.</p>
+
+
+<h4>NEST SITE</h4>
+
+<p>In 1945 the nest was situated on a metal-covered cement ledge, two feet
+wide and 48 feet above the ground, at the northeast corner of the Museum
+Building. The nest was protected on the east by a stone abutment of the
+building and on the south by the north wall of the building itself. Here
+the nest could be observed at will through a laboratory window without
+disturbing the birds. The taking of notes was begun at the time of
+egg-laying and extended to the time at which the young left the nest,
+February 3 through April 26, 1945. In 1946 the owls nested farther down
+the north side of the building, behind two cement pillars, approximately
+25 feet above the ground. To examine the nest in 1946 it was necessary
+to lower an observer down the side of the building by means of a rope.
+Observations of this nest were never made more frequently than every
+other day. The adult owls were first seen at the nest on February 3,
+1946; careful examination of the nest began when the one egg hatched on
+March 7 and continued until April 25, shortly before the young owl left
+the nest.</p>
+
+<p>One large cottonwood tree, used by the parent-owls as a landing place
+whenever they were forced from the nest, was situated approximately 110
+feet to the north and a five-story building was located 80 feet farther
+to the north. Numerous smaller trees line<span class='pagenum'><a name="Page_160" id="Page_160">[Pg 160]</a></span> the street to the east and
+there are some on the lawns around the Museum. Also, there are about two
+acres of trees 225 feet west of the nest-site where the parent-owls took
+refuge when forced from the cottonwood tree.</p>
+
+<p>The nest, if it can be called a nest, was no more than a few bare
+branches of the Virginia creeper, which covers the side of the building,
+together with some excrement which the owls tended to push to the
+periphery of the nest. For most of the time the three eggs in 1945 lay
+directly on the metal which covered the ledge, because there was no
+definite floor to the nest. The single egg in 1946 lay on the cement
+shelf between the pillars and the wall of the building. This laxity in
+nest building by Great Horned Owls apparently is not uncommon (see Bent,
+1938:300).</p>
+
+
+<h4>PERIOD OF INCUBATION</h4>
+
+<p>Incubation of the eggs probably began, in 1945, on February 5, the day
+the first egg was laid. It has usually been assumed that, in birds of
+prey, incubation begins when the first egg is laid. The last of the
+three eggs was laid February 7. In 1946, the single egg was being
+incubated on February 4. Since another egg had been laid two or three
+days before thisa broken egg was found beneath the nest and there were
+remnants of the egg in the nestincubation may have started as early as
+February 1 or February 2. In comparing these dates of initial incubation
+with other recorded dates of nesting, only those from places at, or
+near, the latitude of Lawrence, Kansas, in the central United States,
+should be expected to be approximately the same since the times of
+egg-laying and incubation are progressively later in the year as
+approach is made toward the polar region. Baumgartner (1938:279) has
+previously pointed this out.</p>
+
+<p>The incubation period for the Great Horned Owl in the central United
+States has usually been regarded as 28 to 29 days. In the nest under
+observation in 1945, two eggs hatched on March 12 and are assumed to be
+the first two eggs laid, with an incubation period for each of 35 and 34
+days, respectively, and the third egg hatched on March 14 with an
+incubation period of 35 days. In 1946, the single egg hatched on the
+33rd day, assuming that incubation began on February 2, for the egg
+hatched March 7. In the period of egg-laying and also in incubation, the
+parent bird in 1945 was frequently disturbed by persons who peered at it
+through the window. Curious observers handled the eggs at least once and
+vigorous<span class='pagenum'><a name="Page_161" id="Page_161">[Pg 161]</a></span> pounding by carpenters in the room adjacent to the nest
+frequently flushed the adult bird but did not cause desertion of the
+nest. It may be that such disturbances prolonged the incubation period.
+However, in 1946, the brooding birds were undisturbed, yet the
+incubation period was nearly as long. If an observer near the nest
+exposed himself in the daytime to the incubating bird, the adult flew,
+but exposure at 50 feet or more from the nest only caused the incubating
+bird to remain alert on the nest. When flushed, the parent usually
+returned to the nest within 15 minutes or less after the observer
+withdrew. On the thirty-second and thirty-third days of incubation in
+1945, the crew of carpenters demolished partitions within the building
+on which the owl was nesting, and within 15 feet of the nest itself. At
+first the adult would fly from the nest at each outburst of hammering
+and, at one time, remained away from the nest for more than two hours.
+After a few hours of intermittent hammering, however, the parent bird
+remained on the nest despite all the noise produced. These observations
+bear out, rather than refute, Baumgartner's statement (1938:281) that
+"the horned owl incubates very closely," for a strong stimulus was
+necessary to keep the owl from covering the eggs.</p>
+
+<p>The egg hatched on March 14, 1945, and approximately two days later than
+the other two, is judged to be the one laid last. This owl, III, was
+always 5 to 21 per cent lighter in weight than the older birds when
+weights for corresponding ages were compared. Whether this difference
+was the result of a lack of food because of dominance of the two older
+birds, or because of a sexual difference, we do not know. The owl that
+hatched in 1946 was likewise markedly lighter than the first two birds
+hatched in 1945 (figure 1). A series of adults from Meade County,
+southwestern Kansas, shows a pronounced secondary sexual difference in
+weight. In this sample the mean weight of 17 males, 1,208 grams, was 21
+per cent less than that of 25 females, 1,531 grams.</p>
+
+
+<h4>GROWTH OF JUVENILES</h4>
+
+<p>The principal measurement of growth taken by us was the weight of the
+owls. In 1945 each of the three owls was weighed daily, with two or
+three exceptions when a 48-hour period was interposed between weighings.
+The young were removed from the nest to a nearby balance, weighed, and
+examined. The owl last hatched (owl III) was weighed on the first day of
+life and on most subsequent days. The other two owls (designated as owls
+I and II)<span class='pagenum'><a name="Page_162" id="Page_162">[Pg 162]</a></span> were first weighed when they were between 53 and 60 hours
+old. On some days the birds were weighed twice, once in the morning and
+once in the late afternoon; on most days, they were weighed only in the
+late afternoon. The owl hatched in 1946 was weighed when seven days old
+and at irregular, but usually two day, intervals thereafter. It was
+weighed always slightly before midday.</p>
+
+<div class="figcenter" style="width: 493px;">
+<img src="images/i_006.jpg" width="493" height="576" alt="Fig. 1. Growth of four Great Horned Owls as shown by
+changes in weight from near the time of hatching until the time of
+leaving the nest." title="" />
+<span class="caption">Fig. 1. Growth of four Great Horned Owls as shown by
+changes in weight from near the time of hatching until the time of
+leaving the nest.</span>
+</div>
+
+<p>The growth of the four owls is well shown by the changes in weight
+recorded in figure 1. For the period during which the young owls
+remained at the nest, growth can be divided into two<span class='pagenum'><a name="Page_163" id="Page_163">[Pg 163]</a></span> phases: (1) a
+rapid increase in weight during the first 3-1/2 or 4 weeks while the
+parent birds are supplying the young with ample food; and (2) a
+subsequent period of slower growth, marked by fluctuations (actual
+losses as well as gains) in weight resulting from the failure of the
+parent birds to provide an ample supply of food. If there is an initial
+period of about one week in postnatal development in which there is a
+rather slow gain in weight, as suggested by Sumner (1933:284), it was
+poorly marked in this instance. Owl IV remained at the nest until the
+50th day of age, and on the 47th and 49th days (not shown on chart, fig.
+1) weighed 1,011.4 grams and 971.4 grams, respectively. By this age, the
+growth curve had definitely flattened out. The fact that owl IV was
+consistently heavier than owl III might be accounted for, in part, by
+the fact that owl IV was always weighed in the morning when it was
+gorged with food. However, Riddle, Charles, and Cauthen (1932) have
+pointed out that when there were two or more pigeons in a nest, each
+grew less rapidly than if there was only one present.</p>
+
+<p>Within about 12 hours after hatching, the smallest of the three owlets
+(III) weighed 48.7 grams. During the first four weeks of postnatal
+growth, each owl gained in weight, daily, an average of 33-1/3 grams or
+an increase of 11.1 per cent. Owl I gained an average of 36.1 grams each
+day, or a daily increase of 10.7 per cent; owl II, 37.8 grams, or 11.2
+per cent; and owl III, 26.1 grams, or 11.4 per cent. From the beginning
+of the fifth week until the time the young left the nest, the three owls
+gained on the average only 12.7 grams or approximately 1.6 per cent in
+weight daily. Individually, the daily mean increases were as follows: I,
+9.6 grams or 0.93 per cent; II, 12.7 grams or 1.86 per cent; III, 15.8
+grams or 1.97 per cent. Prior to the twenty-sixth to twenty-eighth day
+of age, there seldom was any loss in weight from day to day, whereas
+after this period, approximately one weight in four was less than on the
+previous day. These data support the earlier statement that during the
+first 3-1/2 or 4 weeks, there is a relatively uniform and rapid increase
+in weight whereas after this period weight fluctuates.</p>
+
+<p>Growth as measured by changes in weight in these young Great Horned Owls
+parallels growth in some other young birds. For example, nestling
+Red-tailed Hawks, as reported upon by Fitch, Swenson, and Tillotson
+(1946:215), increase in weight rapidly for about the first three weeks
+and then more gradually. Sumner's (1929b) graphs indicate the same
+pattern of growth in the Barn and Great Horned owls and Red-tailed and
+Cooper hawks. Pigeons, judging<span class='pagenum'><a name="Page_164" id="Page_164">[Pg 164]</a></span> from the growth curves for bodily weight
+as given by Riddle, Charles, and Cauthen (1932), increase in weight
+rapidly until somewhere between the twenty-fourth and thirty-second day
+of postnatal development. However, in the Golden Eagle, the early part
+of postnatal development is not one of rapid growth, judging from
+Sumner's diagram (1929a:164), but after the fourth week there is a rapid
+increase in weight. Graphs that Sumner (1929b) gives for Sparrow Hawks,
+Long-eared Owls, and Screech Owls, indicate that in these instances also
+the increase in weight during the first few days of postnatal
+development was not so rapid as it was after the end of the first week.
+Stoner (1935) indicates that in the young Barn Swallow, increase in
+weight was most rapid between the fourth and tenth days, with the young
+remaining at the nest until the twentieth day. Much the same pattern of
+weight increase was noted by Stoner (1945) in the Cliff Swallow.
+Huggins' (1940:228) sigmoid curve for increase in weight in the House
+Wren indicates that the period of rapid growth in this species does not
+begin until the second day. Sumner (1934:284) cites other studies which
+he believes, for altricial birds, indicate three periods of growth, an
+initial period of rather slow gain, a period of maximum increase in
+weight, and a final period of fluctuations. As previously indicated, for
+the Great Horned Owls under observation, and in some other species as
+indicated by published growth curves, the initial period of slow gain is
+lacking.</p>
+
+<p>The period of a decelerated rate of growth in the young Great Horned
+Owls is correlated with the occasional lack of food. The parent birds,
+during this latter period, remain off the nest more of the time during
+the day, and their failure to provide the young with food may represent
+an attempt to force the young to become proficient in flight or to force
+them away from the nest site, which amounts to the same thing. When only
+slightly more than a month old, the young began to test their wings,
+springing into the air, and, in general, becoming more active and alert.
+Sumner (1929b:110) has suggested some other possible reasons for the
+period of decelerated rate of growth.</p>
+
+<p>Although there was a daily increase in weight in the early stages of
+growth, there was a decided fluctuation in any twenty-four hour period.
+On any given day, the young always were heavier in the morning than in
+the afternoon (see figure 2); presumably they were gorged with food
+early in the morning.<span class='pagenum'><a name="Page_165" id="Page_165">[Pg 165]</a></span></p>
+
+<div class="figcenter" style="width: 650px;">
+<img src="images/i_009.jpg" width="650" height="356" alt="Fig. 2. Morning and afternoon weights of two Great Horned
+Owls. Note that in the morning the owls weigh more than in the
+afternoon." title="" />
+<span class="caption">Fig. 2. Morning and afternoon weights of two Great Horned
+Owls. Note that in the morning the owls weigh more than in the
+afternoon.</span>
+</div>
+
+<p>When the young left the nest, they were approximately three-fourths
+grown. When owl I on the 44th day and owl II on the 45th day left the
+nest, they weighed 1,120 and 1,139 grams, respectively, or 73 and 74 per
+cent, respectively, of the average weight of 25 females (1,530.9 grams).
+Owl III weighed 943.3 grams on the 43rd day and owl IV weighed 971.4
+grams on the 49th day, or 78 and 80 per cent, respectively, of the
+average weight of 17 mature males (1,207.7 grams). Owl I left the nest
+18 hours before owl II did. Owl III attempted to leave when 43 days old,
+but for it co&ouml;rdinated flight was impossible and the bird landed on the
+lawn after a 150-foot glide. When attempting to take owl IV from the
+nest on the 49th day, it sprang into the air and by gliding, aided by an
+occasional flap, sailed more than 300 feet before alighting on the
+ground. After we returned the owl to the nest, it immediately sailed
+forth for even a longer distance. When attempt was made to pick up owls
+III and IV after they had flown down to the ground, they rolled over on
+their backs and used both claws and beaks defensively. Such a reaction
+never was noted at the nest; there our hands were inspected, and
+sometimes bitten by the owls as possible sources of food, but the claws
+were rarely used offensively or defensively.</p>
+
+<p>Slightly elevated remiges and rectrices, still in the sheath, were
+visible on the ninth day. Some remiges first ruptured the feather sheath
+on the 14th day; nearly all of the primaries ruptured the sheaths by the
+19th day and the secondaries by the 20th day. The<span class='pagenum'><a name="Page_166" id="Page_166">[Pg 166]</a></span> amount of eruption
+from the sheath for primaries, secondaries, and rectrices, as given in
+table 1, was determined by measuring the one feather of, say, the
+secondaries, judged to be near the mean in degree of eruption. The
+feathers of the wing at 21 and 47 days of age are shown in figure 5. On
+the eighth day, or slightly before, the white nestling down of the newly
+hatched bird was replaced by a downy immature plumage, which was more
+yellowish than the preceding plumage. The development of the plumage in
+the birds under observation was much the same as that recorded by Sumner
+(1933) in <i>Bubo virginianus pacificus</i> Cassin.</p>
+
+
+<h4><span class="smcap">Table 1.</span>Changes with age in certain parts of a young Great Horned Owl
+hatched in 1946.</h4>
+
+<h4>(Measurements are in millimeters)</h4>
+
+
+
+<div class='center'>
+<table border="1" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Age in days</td><td align='left'> 19</td><td align='left'> 21</td><td align='left'> 26</td><td align='left'> 33</td><td align='left'> 37</td><td align='left'> 39</td><td align='left'> 49</td></tr>
+<tr><td align='left'>Length of erupted portion of "average" primary</td><td align='left'> 6.0</td><td align='left'> 10.0</td><td align='left'> 26.0</td><td align='left'> 93.0</td><td align='left'> 87.5</td><td align='left'> 99.2</td><td align='left'>......</td></tr>
+<tr><td align='left'>Length of erupted portion of "average" secondary</td><td align='left'>......</td><td align='left'> 5.0</td><td align='left'> 25.0</td><td align='left'> 60.0</td><td align='left'> 78.0</td><td align='left'> 95.0</td><td align='left'>......</td></tr>
+<tr><td align='left'>Length of erupted portion of "average" rectrix</td><td align='left'> 17.0</td><td align='left'>......</td><td align='left'> 16.0</td><td align='left'> 28.0</td><td align='left'>......</td><td align='left'>......</td><td align='left'> 78.0</td></tr>
+<tr><td align='left'>Length of exposed culmen without cere</td><td align='left'>......</td><td align='left'> 19.6</td><td align='left'>......</td><td align='left'> 22.5</td><td align='left'>......</td><td align='left'> 24.0</td><td align='left'> 23.7</td></tr>
+<tr><td align='left'>Length of total culmen</td><td align='left'>......</td><td align='left'> 30.4</td><td align='left'>......</td><td align='left'> 36.0</td><td align='left'>......</td><td align='left'> 38.5</td><td align='left'> 40.0</td></tr>
+<tr><td align='left'>Length of femur</td><td align='left'> 69.0</td><td align='left'>......</td><td align='left'>......</td><td align='left'> 87.5</td><td align='left'>......</td><td align='left'> 89.5</td><td align='left'>......</td></tr>
+</table></div>
+
+
+<p>Traces of the egg-tooth were retained until the ninth day in two owls
+and until the 11th day in another. In owl IV, the egg-tooth was lost
+sometime between the 9th and 14th day. Changes in the length of the
+culmen are indicated in Table 1. The length of total culmen of owl IV
+when 47 days old is slightly greater than the average for three adult
+male owls from Lawrence, Kansas (40.0 mm. as contrasted with 39.2 mm. in
+the adults). The length of exposed culmen, without cere, in the same
+bird when 47 days old is less than the average of this measurement in
+the three adults from Lawrence (23.7 mm. as contrasted with 26.5 mm. in
+the adults). The femur was measured on three occasions as accurately as
+possible<span class='pagenum'><a name="Page_167" id="Page_167">[Pg 167]</a></span> through the skin and flesh. The precise boundaries of the
+femur could not be determined and the thickness of the skin and certain
+muscle is included. These measurements are not given to indicate actual
+length of the femur, but to indicate the relative changes in length of
+this bone.</p>
+
+<p>Remnants of the yolk stalk were clearly evident at seven days of age
+(see fig. 5) in the owl hatched in 1946 and were still present when the
+owl was last examined (49 days of age) just before the young left the
+nest. The scablike remnant was not noted in the three young hatched in
+1945, but close inspection was not made to see if it was present.</p>
+
+<p>The instinctive reactions of young horned owls shortly after hatching
+have been fully described by Sumner (1934). By the third day our owls
+could raise their heads, but when a bird was undisturbed its head lay on
+the nest floor and the wings were slightly spread. The eyes of owls I
+and II opened at about 7-1/2 days, those of owl III on the 6th day, and
+those of owl IV sometime between the 7th and 9th days. After the eyes
+opened, the head was held erect more of the time. The young responded
+with "cries" when disturbed by handling, when stimulated by certain
+movements of the parent, or by movements of our hands near their heads,
+which suggested to them the possibility of being fed. Cries were evident
+but weak in the unhatched, pipped, egg, but soon after hatching
+increased in intensity, and beginning at six days of age were replaced,
+in part, by the characteristic "bill-snapping" of more mature birds.
+These cries of the young may serve, among other things, for recognition,
+inasmuch as they were given when the parent was inspecting the young.
+When the parent returned to the nest and covered the young, after having
+been flushed, it sometimes uttered a special note, "hut, hut, hut," much
+like the "cluck" note of the hen of the domestic chicken. The young
+responded to these notes with faint cries, in contrast to the loud cries
+signifying alarm and possibly hunger, which they gave when the parents
+were absent from the nest.</p>
+
+<p>The first definite evidence that the young were attempting to feed
+themselves was obtained when they were 23 days old. Frequently
+thereafter, fresh blood was found on their beaks and claws, but as late
+as the 34th day a parent was seen feeding them. That day, after being
+flushed, a parent returned to the nest at 7 p. m., and began tearing
+away parts of a cottontail which had previously been brought to the
+nest. Bones in the hind leg of the rabbit broke<span class='pagenum'><a name="Page_168" id="Page_168">[Pg 168]</a></span> readily under pressure
+of the parent's bill, and the three young crowded in close, opening
+their bills widely and placing them around that of the parent. Of
+cottontails, the only parts consistently uneaten were the upper cheek
+teeth and the supporting maxillae and connecting palatal bridge.</p>
+
+
+<h4>FOOD BROUGHT TO THE NEST</h4>
+
+<p>In the 45 days that the young remained at the nest site in 1945,
+ninety-one individuals of 16 different species of animals were brought
+by the parent owls (table 2). Probably a few smaller animals, of which
+we saw no traces, were caught and eaten at night. In 1946, two
+additional kinds of birds were brought to the nest: 1 Baldpate (<i>Mareca
+americana</i>) and 1 Pied-billed Grebe (<i>Podilymbus podiceps</i>). The large
+number of Rock Doves in the list can be explained by their abundance on
+the buildings on the University campus, including the Museum building
+where some were nesting as close as 100 feet to the owl nest. When the
+owls were less than a week old, only small birds and mammals were
+brought (young Rock Doves, Robins, Starlings, Grasshopper Sparrow,
+meadow mouse, and Norway rat). The first rabbit was brought when the
+owls were eight days old.</p>
+
+
+<h4><span class="smcap">Table 2.</span> Number of food items brought to the nest by the Great Horned
+Owls in 1945</h4>
+
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td colspan="2">Birds</td></tr>
+<tr><td align='left'>Rock Dove (<i>Columba livia</i>)</td><td align='right'>32</td></tr>
+<tr><td align='left'>Robin (<i>Turdus migratorius</i>)</td><td align='right'>6</td></tr>
+<tr><td align='left'>Starling (<i>Sturnus vulgaris</i>)</td><td align='right'>4</td></tr>
+<tr><td align='left'>Mourning Dove (<i>Zenaidura macroura</i>)</td><td align='right'>10</td></tr>
+<tr><td align='left'>Meadowlark (<i>Sturnella sp.</i>)</td><td align='right'>3</td></tr>
+<tr><td align='left'>Red-wing (<i>Agelaius phoeniceus</i>)</td><td align='right'>1</td></tr>
+<tr><td align='left'>Bronzed Grackle (<i>Quiscalus versicolor</i>)</td><td align='right'>1</td></tr>
+<tr><td align='left'>Mockingbird (<i>Mimus polyglottos</i>)</td><td align='right'>1</td></tr>
+<tr><td align='left'>Brown Thrasher (<i>Toxostoma rufum</i>)</td><td align='right'>1</td></tr>
+<tr><td align='left'>Grasshopper Sparrow (<i>Ammodramus savannarum</i>)</td><td align='right'>1</td></tr>
+<tr><td align='left'>Coot (<i>Fulica americana</i>)</td><td align='right'>3</td></tr>
+<tr><td align='left'>Sora (<i>Porzana carolina</i>)</td><td align='right'>1</td></tr>
+<tr><td align='left'>Blue-winged Teal (<i>Anas discors</i>)</td><td align='right'>1</td></tr>
+<tr><td colspan="2">Mammals</td></tr>
+<tr><td align='left'><i>Sylvilagus floridanus</i></td><td align='right'>19</td></tr>
+<tr><td align='left'><i>Rattus norvegicus</i></td><td align='right'>6</td></tr>
+<tr><td align='left'><i>Microtus ochrogaster</i></td><td align='right'>1</td></tr>
+</table></div>
+
+<p>After the 28th day, only 18 food items, or slightly less than 20 per
+cent of the total number, were brought to the nest. These last 18 food
+items brought after the owls were 4 weeks old were no larger or bulkier
+than those brought in the previous 20 days. The beginning of this period
+of reduced amount of food corresponds to<span class='pagenum'><a name="Page_169" id="Page_169">[Pg 169]</a></span> the beginning of the second
+phase of growth characterized by marked fluctuations in weight.</p>
+
+<p>Fox squirrels (<i>Sciurus niger</i>) are abundant on the University campus,
+yet there were no remains of this mammal at the nest. This may be
+explained by the fact that fox squirrels are principally diurnal and
+Great Horned Owls feed principally at night. Yet in early February,
+1946, when the owls were preparing the nest, they frequently flew on and
+off the nest in the early twilight of evening while one or two fox
+squirrels fed in the periphery of trees not more than 25 feet away. Yet
+the owls flew off to the west and left this source of food unmolested.</p>
+
+<p>Whether both owls regularly attended the young we do not know, for the
+adults were not distinctively marked. On March 17, 1945, when weighing
+the young, one parent bird started to return to the nest but was
+frightened away by the observer who at the same time noted the other
+parent perched in an adjacent tree. This was the first time two adults
+were seen at the same time near the nest. In 1946, two adult owls
+(presumably both were parents) were within sight at one time when the
+young owl first sailed forth and landed in a wooded area some 100 yards
+away.</p>
+
+
+<h4>SUMMARY</h4>
+
+<p>Great Horned Owls (<i>Bubo virginianus virginianus</i>) have employed as nest
+sites the protruding shelves of the stone wall of the Museum of Natural
+History at the University of Kansas for several years. In 1945, daily
+observations were made on one such nest and its three young, and in 1946
+irregular observations were made on another such nest and the one young
+owl. The incubation time for the three owls, hatched in 1945, was 35
+days for two of the young and 34 days for the third; for the one owl
+hatched in 1946, the incubation time was at least 33 days. Two owls were
+consistently smaller; when these smaller two left the nest they were,
+respectively, 21 and 17 per cent lighter than the other two. The smaller
+two were judged to be males because adult males in Kansas average
+smaller by 21 per cent than adult females.</p>
+
+<p>Growth of the nestling young is divisible into (1) a period of rapid
+increase in weight during the first 25 to 28 days, and (2) a subsequent
+period marked by gains and losses in weight. The fluctuations in this
+latter period are correlated with a reduction in food brought to the
+nest by the parent birds and with the development of habits of flight.
+This second period may be considered to be a<span class='pagenum'><a name="Page_170" id="Page_170">[Pg 170]</a></span> period of "weaning." By
+the forty-fifth day, the young owls are able to fly short distances and
+thus are able to leave the site of the nest permanently. At this time
+they are about three-fourths grown.</p>
+
+<p>Ninety-one individuals of 16 species of birds and mammals made up the
+food items brought to the nest in 1945. Two factors seem to be concerned
+in the acquisition of prey: (1) its availability and (2) appropriate
+size of the prey.</p>
+
+
+
+<hr style="width: 65%;" />
+<h2>LITERATURE CITED</h2>
+
+<p><span class="smcap">Baumgartner, F. M.</span></p>
+
+<p>1938. Courtship and nesting of the Great Horned Owls. Wilson Bull.,
+50:274-285.</p>
+
+
+<p><span class="smcap">Bent, A. C.</span></p>
+
+<p>1938. Life histories of North American birds of prey (Part 2), Orders
+Falconiformes and Strigiformes. U. S. Nat. Mus., Bull. 170, viii + 482
+pp.</p>
+
+
+<p><span class="smcap">Fitch, H. S.</span>, <span class="smcap">Swenson, F.</span>, and <span class="smcap">Tillotson, D. F.</span></p>
+
+<p>1946. Behavior and food habits of the Red-tailed Hawk. Condor,
+48:205-237.</p>
+
+
+<p><span class="smcap">Huggins, S. E.</span></p>
+
+<p>1940. Relative growth in the House Wren. Growth, 4:225-236.</p>
+
+
+<p><span class="smcap">Riddle, O.</span>, <span class="smcap">Charles, D. R.</span>, and <span class="smcap">Cauthen, G. E.</span></p>
+
+<p>1932. Relative growth rates in large and small races of pigeons. Proc.
+Soc. Exp. Biol. and Med., 29:1216-1220.</p>
+
+
+<p><span class="smcap">Stoner, D.</span></p>
+
+<p>1935. Temperature and growth studies on the Barn Swallow. Auk,
+52:400-407.</p>
+
+<p>1945. Temperature and growth studies of the Northern Cliff Swallow. Auk,
+62:207-216.</p>
+
+
+<p><span class="smcap">Sumner, E. L., Jr.</span></p>
+
+<p>1929a. Notes on the growth and behavior of young Golden Eagles. Auk,
+46:161-169.</p>
+
+<p>1929b. Comparative studies in the growth of young raptores. Condor,
+31:85-111.</p>
+
+<p>1933. The growth of some young raptorial birds. Univ. California Publ.
+Zo&ouml;l., 40:277-307.</p>
+
+<p>1934. The behavior of some young raptorial birds. Univ. California Publ.
+Zo&ouml;l., 40:331-361.</p>
+
+<hr style="width: 65%;" /><p><span class='pagenum'><a name="Page_171" id="Page_171">[Pg 171]</a></span></p>
+
+<div class="figcenter" style="width: 650px;">
+<img src="images/i_015.jpg" width="650" height="464" alt="Fig. 3. Young Great Horned Owls in nest. Two owls are 7,
+12, 18, 32, and 36 days of age, respectively; the third owl is about 2
+days younger in each instance." title="" />
+<span class="caption">Fig. 3. Young Great Horned Owls in nest. Two owls are 7,
+12, 18, 32, and 36 days of age, respectively; the third owl is about 2
+days younger in each instance.</span>
+</div>
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_172" id="Page_172">[Pg 172]</a></span></p>
+
+<div class="figcenter" style="width: 390px;">
+<img src="images/i_016.jpg" width="390" height="650" alt="Fig. 4. Young Great Horned Owl hatched in 1946. The two
+lower pictures show the developing facial mask. Photographs by Jo&atilde;o
+Moojen." title="" />
+<span class="caption">Fig. 4. Young Great Horned Owl hatched in 1946. The two
+lower pictures show the developing facial mask. Photographs by Jo&atilde;o
+Moojen.</span>
+</div>
+
+<hr style="width: 65%;" /><p><span class='pagenum'><a name="Page_173" id="Page_173">[Pg 173]</a></span></p>
+<div class="figcenter" style="width: 480px;">
+<img src="images/i_017.jpg" width="480" height="650" alt="Fig. 5. Young Great Horned Owl hatched in 1946. Upper
+row: Ventral views showing scar of yolk sac and ventral side of wing.
+Middle row: Ventral (left) and dorsal view of wing at 21 days. Bottom
+row: Ventral (left) and dorsal view of wing at 47 days. Photographs by
+Jo&atilde;o Moojen." title="" />
+<span class="caption">Fig. 5. Young Great Horned Owl hatched in 1946. Upper
+row: Ventral views showing scar of yolk sac and ventral side of wing.
+Middle row: Ventral (left) and dorsal view of wing at 21 days. Bottom
+row: Ventral (left) and dorsal view of wing at 47 days. Photographs by
+Jo&atilde;o Moojen.</span>
+</div>
+
+
+<h4>21-6958</h4>
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of the Project Gutenberg EBook of The Postnatal Development of Two
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+</body>
+</html>
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@@ -0,0 +1,962 @@
+The Project Gutenberg EBook of The Postnatal Development of Two Broods of
+Great Horned Owls, by Donald F. Hoffmeister and Henry W. Setzer
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: The Postnatal Development of Two Broods of Great Horned Owls
+       Bubo virginianus
+
+Author: Donald F. Hoffmeister
+        Henry W. Setzer
+
+Release Date: January 31, 2011 [EBook #35118]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE POSTNATAL DEVELOPMENT OF ***
+
+
+
+
+Produced by Chris Curnow, Joseph Cooper, Josephine Paolucci
+and the Online Distributed Proofreading Team at
+http://www.pgdp.net.
+
+
+
+
+
+
+
+THE POSTNATAL DEVELOPMENT OF TWO BROODS OF GREAT HORNED OWLS
+
+(Bubo virginianus)
+
+BY
+
+DONALD F. HOFFMEISTER AND HENRY W. SETZER
+
+University of Kansas Publications
+
+Museum of Natural History
+
+Volume 1, No. 8, pp. 157-173
+October 6, 1947
+
+UNIVERSITY OF KANSAS
+LAWRENCE
+1947
+
+
+UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY
+Editors: E. Raymond Hall, Chairman, Donald S. Farner, H. H. Lane,
+Edward H. Taylor
+
+Volume 1, No. 8, pp. 157-173
+October 6, 1947
+
+UNIVERSITY OF KANSAS
+Lawrence, Kansas
+
+PRINTED BY
+FERD VOILAND, JR., STATE PRINTER
+TOPEKA, KANSAS
+1947
+
+21-6958
+
+
+
+
+The Postnatal Development of Two Broods of Great Horned Owls
+
+(_Bubo virginianus_)
+
+By
+
+DONALD F. HOFFMEISTER AND HENRY W. SETZER
+
+
+Opportunity regularly to observe at the nest the development of young
+Great Horned Owls, _Bubo virginianus_ (Gmelin), under favorable
+conditions, was afforded when a pair nested and reared their three
+offspring in 1945 and one offspring in 1946 on the vine-covered north
+wall of the Museum of Natural History at the University of Kansas. The
+observations here reported are based primarily on the three young raised
+in 1945 when daily observations were made. These have been supplemented
+by other observations made of the one nestling in 1946. Unless otherwise
+stated, observations pertain to the nest and three young in 1945.
+
+
+NEST SITE
+
+In 1945 the nest was situated on a metal-covered cement ledge, two feet
+wide and 48 feet above the ground, at the northeast corner of the Museum
+Building. The nest was protected on the east by a stone abutment of the
+building and on the south by the north wall of the building itself. Here
+the nest could be observed at will through a laboratory window without
+disturbing the birds. The taking of notes was begun at the time of
+egg-laying and extended to the time at which the young left the nest,
+February 3 through April 26, 1945. In 1946 the owls nested farther down
+the north side of the building, behind two cement pillars, approximately
+25 feet above the ground. To examine the nest in 1946 it was necessary
+to lower an observer down the side of the building by means of a rope.
+Observations of this nest were never made more frequently than every
+other day. The adult owls were first seen at the nest on February 3,
+1946; careful examination of the nest began when the one egg hatched on
+March 7 and continued until April 25, shortly before the young owl left
+the nest.
+
+One large cottonwood tree, used by the parent-owls as a landing place
+whenever they were forced from the nest, was situated approximately 110
+feet to the north and a five-story building was located 80 feet farther
+to the north. Numerous smaller trees line the street to the east and
+there are some on the lawns around the Museum. Also, there are about two
+acres of trees 225 feet west of the nest-site where the parent-owls took
+refuge when forced from the cottonwood tree.
+
+The nest, if it can be called a nest, was no more than a few bare
+branches of the Virginia creeper, which covers the side of the building,
+together with some excrement which the owls tended to push to the
+periphery of the nest. For most of the time the three eggs in 1945 lay
+directly on the metal which covered the ledge, because there was no
+definite floor to the nest. The single egg in 1946 lay on the cement
+shelf between the pillars and the wall of the building. This laxity in
+nest building by Great Horned Owls apparently is not uncommon (see Bent,
+1938:300).
+
+
+PERIOD OF INCUBATION
+
+Incubation of the eggs probably began, in 1945, on February 5, the day
+the first egg was laid. It has usually been assumed that, in birds of
+prey, incubation begins when the first egg is laid. The last of the
+three eggs was laid February 7. In 1946, the single egg was being
+incubated on February 4. Since another egg had been laid two or three
+days before this--a broken egg was found beneath the nest and there were
+remnants of the egg in the nest--incubation may have started as early as
+February 1 or February 2. In comparing these dates of initial incubation
+with other recorded dates of nesting, only those from places at, or
+near, the latitude of Lawrence, Kansas, in the central United States,
+should be expected to be approximately the same since the times of
+egg-laying and incubation are progressively later in the year as
+approach is made toward the polar region. Baumgartner (1938:279) has
+previously pointed this out.
+
+The incubation period for the Great Horned Owl in the central United
+States has usually been regarded as 28 to 29 days. In the nest under
+observation in 1945, two eggs hatched on March 12 and are assumed to be
+the first two eggs laid, with an incubation period for each of 35 and 34
+days, respectively, and the third egg hatched on March 14 with an
+incubation period of 35 days. In 1946, the single egg hatched on the
+33rd day, assuming that incubation began on February 2, for the egg
+hatched March 7. In the period of egg-laying and also in incubation, the
+parent bird in 1945 was frequently disturbed by persons who peered at it
+through the window. Curious observers handled the eggs at least once and
+vigorous pounding by carpenters in the room adjacent to the nest
+frequently flushed the adult bird but did not cause desertion of the
+nest. It may be that such disturbances prolonged the incubation period.
+However, in 1946, the brooding birds were undisturbed, yet the
+incubation period was nearly as long. If an observer near the nest
+exposed himself in the daytime to the incubating bird, the adult flew,
+but exposure at 50 feet or more from the nest only caused the incubating
+bird to remain alert on the nest. When flushed, the parent usually
+returned to the nest within 15 minutes or less after the observer
+withdrew. On the thirty-second and thirty-third days of incubation in
+1945, the crew of carpenters demolished partitions within the building
+on which the owl was nesting, and within 15 feet of the nest itself. At
+first the adult would fly from the nest at each outburst of hammering
+and, at one time, remained away from the nest for more than two hours.
+After a few hours of intermittent hammering, however, the parent bird
+remained on the nest despite all the noise produced. These observations
+bear out, rather than refute, Baumgartner's statement (1938:281) that
+"the horned owl incubates very closely," for a strong stimulus was
+necessary to keep the owl from covering the eggs.
+
+The egg hatched on March 14, 1945, and approximately two days later than
+the other two, is judged to be the one laid last. This owl, III, was
+always 5 to 21 per cent lighter in weight than the older birds when
+weights for corresponding ages were compared. Whether this difference
+was the result of a lack of food because of dominance of the two older
+birds, or because of a sexual difference, we do not know. The owl that
+hatched in 1946 was likewise markedly lighter than the first two birds
+hatched in 1945 (figure 1). A series of adults from Meade County,
+southwestern Kansas, shows a pronounced secondary sexual difference in
+weight. In this sample the mean weight of 17 males, 1,208 grams, was 21
+per cent less than that of 25 females, 1,531 grams.
+
+
+GROWTH OF JUVENILES
+
+The principal measurement of growth taken by us was the weight of the
+owls. In 1945 each of the three owls was weighed daily, with two or
+three exceptions when a 48-hour period was interposed between weighings.
+The young were removed from the nest to a nearby balance, weighed, and
+examined. The owl last hatched (owl III) was weighed on the first day of
+life and on most subsequent days. The other two owls (designated as owls
+I and II) were first weighed when they were between 53 and 60 hours
+old. On some days the birds were weighed twice, once in the morning and
+once in the late afternoon; on most days, they were weighed only in the
+late afternoon. The owl hatched in 1946 was weighed when seven days old
+and at irregular, but usually two day, intervals thereafter. It was
+weighed always slightly before midday.
+
+[Illustration: FIG. 1. Growth of four Great Horned Owls as shown by
+changes in weight from near the time of hatching until the time of
+leaving the nest.]
+
+The growth of the four owls is well shown by the changes in weight
+recorded in figure 1. For the period during which the young owls
+remained at the nest, growth can be divided into two phases: (1) a
+rapid increase in weight during the first 3-1/2 or 4 weeks while the
+parent birds are supplying the young with ample food; and (2) a
+subsequent period of slower growth, marked by fluctuations (actual
+losses as well as gains) in weight resulting from the failure of the
+parent birds to provide an ample supply of food. If there is an initial
+period of about one week in postnatal development in which there is a
+rather slow gain in weight, as suggested by Sumner (1933:284), it was
+poorly marked in this instance. Owl IV remained at the nest until the
+50th day of age, and on the 47th and 49th days (not shown on chart, fig.
+1) weighed 1,011.4 grams and 971.4 grams, respectively. By this age, the
+growth curve had definitely flattened out. The fact that owl IV was
+consistently heavier than owl III might be accounted for, in part, by
+the fact that owl IV was always weighed in the morning when it was
+gorged with food. However, Riddle, Charles, and Cauthen (1932) have
+pointed out that when there were two or more pigeons in a nest, each
+grew less rapidly than if there was only one present.
+
+Within about 12 hours after hatching, the smallest of the three owlets
+(III) weighed 48.7 grams. During the first four weeks of postnatal
+growth, each owl gained in weight, daily, an average of 33-1/3 grams or
+an increase of 11.1 per cent. Owl I gained an average of 36.1 grams each
+day, or a daily increase of 10.7 per cent; owl II, 37.8 grams, or 11.2
+per cent; and owl III, 26.1 grams, or 11.4 per cent. From the beginning
+of the fifth week until the time the young left the nest, the three owls
+gained on the average only 12.7 grams or approximately 1.6 per cent in
+weight daily. Individually, the daily mean increases were as follows: I,
+9.6 grams or 0.93 per cent; II, 12.7 grams or 1.86 per cent; III, 15.8
+grams or 1.97 per cent. Prior to the twenty-sixth to twenty-eighth day
+of age, there seldom was any loss in weight from day to day, whereas
+after this period, approximately one weight in four was less than on the
+previous day. These data support the earlier statement that during the
+first 3-1/2 or 4 weeks, there is a relatively uniform and rapid increase
+in weight whereas after this period weight fluctuates.
+
+Growth as measured by changes in weight in these young Great Horned Owls
+parallels growth in some other young birds. For example, nestling
+Red-tailed Hawks, as reported upon by Fitch, Swenson, and Tillotson
+(1946:215), increase in weight rapidly for about the first three weeks
+and then more gradually. Sumner's (1929b) graphs indicate the same
+pattern of growth in the Barn and Great Horned owls and Red-tailed and
+Cooper hawks. Pigeons, judging from the growth curves for bodily weight
+as given by Riddle, Charles, and Cauthen (1932), increase in weight
+rapidly until somewhere between the twenty-fourth and thirty-second day
+of postnatal development. However, in the Golden Eagle, the early part
+of postnatal development is not one of rapid growth, judging from
+Sumner's diagram (1929a:164), but after the fourth week there is a rapid
+increase in weight. Graphs that Sumner (1929b) gives for Sparrow Hawks,
+Long-eared Owls, and Screech Owls, indicate that in these instances also
+the increase in weight during the first few days of postnatal
+development was not so rapid as it was after the end of the first week.
+Stoner (1935) indicates that in the young Barn Swallow, increase in
+weight was most rapid between the fourth and tenth days, with the young
+remaining at the nest until the twentieth day. Much the same pattern of
+weight increase was noted by Stoner (1945) in the Cliff Swallow.
+Huggins' (1940:228) sigmoid curve for increase in weight in the House
+Wren indicates that the period of rapid growth in this species does not
+begin until the second day. Sumner (1934:284) cites other studies which
+he believes, for altricial birds, indicate three periods of growth, an
+initial period of rather slow gain, a period of maximum increase in
+weight, and a final period of fluctuations. As previously indicated, for
+the Great Horned Owls under observation, and in some other species as
+indicated by published growth curves, the initial period of slow gain is
+lacking.
+
+The period of a decelerated rate of growth in the young Great Horned
+Owls is correlated with the occasional lack of food. The parent birds,
+during this latter period, remain off the nest more of the time during
+the day, and their failure to provide the young with food may represent
+an attempt to force the young to become proficient in flight or to force
+them away from the nest site, which amounts to the same thing. When only
+slightly more than a month old, the young began to test their wings,
+springing into the air, and, in general, becoming more active and alert.
+Sumner (1929b:110) has suggested some other possible reasons for the
+period of decelerated rate of growth.
+
+Although there was a daily increase in weight in the early stages of
+growth, there was a decided fluctuation in any twenty-four hour period.
+On any given day, the young always were heavier in the morning than in
+the afternoon (see figure 2); presumably they were gorged with food
+early in the morning.
+
+[Illustration: FIG. 2. Morning and afternoon weights of two Great Horned
+Owls. Note that in the morning the owls weigh more than in the
+afternoon.]
+
+When the young left the nest, they were approximately three-fourths
+grown. When owl I on the 44th day and owl II on the 45th day left the
+nest, they weighed 1,120 and 1,139 grams, respectively, or 73 and 74 per
+cent, respectively, of the average weight of 25 females (1,530.9 grams).
+Owl III weighed 943.3 grams on the 43rd day and owl IV weighed 971.4
+grams on the 49th day, or 78 and 80 per cent, respectively, of the
+average weight of 17 mature males (1,207.7 grams). Owl I left the nest
+18 hours before owl II did. Owl III attempted to leave when 43 days old,
+but for it cooerdinated flight was impossible and the bird landed on the
+lawn after a 150-foot glide. When attempting to take owl IV from the
+nest on the 49th day, it sprang into the air and by gliding, aided by an
+occasional flap, sailed more than 300 feet before alighting on the
+ground. After we returned the owl to the nest, it immediately sailed
+forth for even a longer distance. When attempt was made to pick up owls
+III and IV after they had flown down to the ground, they rolled over on
+their backs and used both claws and beaks defensively. Such a reaction
+never was noted at the nest; there our hands were inspected, and
+sometimes bitten by the owls as possible sources of food, but the claws
+were rarely used offensively or defensively.
+
+Slightly elevated remiges and rectrices, still in the sheath, were
+visible on the ninth day. Some remiges first ruptured the feather sheath
+on the 14th day; nearly all of the primaries ruptured the sheaths by the
+19th day and the secondaries by the 20th day. The amount of eruption
+from the sheath for primaries, secondaries, and rectrices, as given in
+table 1, was determined by measuring the one feather of, say, the
+secondaries, judged to be near the mean in degree of eruption. The
+feathers of the wing at 21 and 47 days of age are shown in figure 5. On
+the eighth day, or slightly before, the white nestling down of the newly
+hatched bird was replaced by a downy immature plumage, which was more
+yellowish than the preceding plumage. The development of the plumage in
+the birds under observation was much the same as that recorded by Sumner
+(1933) in _Bubo virginianus pacificus_ Cassin.
+
+
+TABLE 1.--Changes with age in certain parts of a young Great Horned Owl
+hatched in 1946.
+
+(Measurements are in millimeters)
+
+==========================+======+======+======+======+======+======+======
+Age in days               |  19  |  21  |  26  |  33  |  37  |  39  |  49
+--------------------------+------+------+------+------+------+------+------
+Length of erupted portion |      |      |      |      |      |      |
+of "average" primary      |  6.0 | 10.0 | 26.0 | 93.0 | 87.5 | 99.2 |......
+--------------------------+------+------+------+------+------+------+------
+Length of erupted portion |      |      |      |      |      |      |
+of "average" secondary    |......|  5.0 | 25.0 | 60.0 | 78.0 | 95.0 |......
+--------------------------+------+------+------+------+------+------+------
+Length of erupted portion |      |      |      |      |      |      |
+of "average" rectrix      | 17.0 |......| 16.0 | 28.0 |......|......| 78.0
+--------------------------+------+------+------+------+------+------+------
+Length of exposed culmen  |      |      |      |      |      |      |
+without cere              |......| 19.6 |......| 22.5 |......| 24.0 | 23.7
+--------------------------+------+------+------+------+------+------+------
+Length of total culmen    |......| 30.4 |......| 36.0 |......| 38.5 | 40.0
+--------------------------+------+------+------+------+------+------+------
+Length of femur           | 69.0 |......|......| 87.5 |......| 89.5 |......
+--------------------------+------+------+------+------+------+------+------
+
+Traces of the egg-tooth were retained until the ninth day in two owls
+and until the 11th day in another. In owl IV, the egg-tooth was lost
+sometime between the 9th and 14th day. Changes in the length of the
+culmen are indicated in Table 1. The length of total culmen of owl IV
+when 47 days old is slightly greater than the average for three adult
+male owls from Lawrence, Kansas (40.0 mm. as contrasted with 39.2 mm. in
+the adults). The length of exposed culmen, without cere, in the same
+bird when 47 days old is less than the average of this measurement in
+the three adults from Lawrence (23.7 mm. as contrasted with 26.5 mm. in
+the adults). The femur was measured on three occasions as accurately as
+possible through the skin and flesh. The precise boundaries of the
+femur could not be determined and the thickness of the skin and certain
+muscle is included. These measurements are not given to indicate actual
+length of the femur, but to indicate the relative changes in length of
+this bone.
+
+Remnants of the yolk stalk were clearly evident at seven days of age
+(see fig. 5) in the owl hatched in 1946 and were still present when the
+owl was last examined (49 days of age) just before the young left the
+nest. The scablike remnant was not noted in the three young hatched in
+1945, but close inspection was not made to see if it was present.
+
+The instinctive reactions of young horned owls shortly after hatching
+have been fully described by Sumner (1934). By the third day our owls
+could raise their heads, but when a bird was undisturbed its head lay on
+the nest floor and the wings were slightly spread. The eyes of owls I
+and II opened at about 7-1/2 days, those of owl III on the 6th day, and
+those of owl IV sometime between the 7th and 9th days. After the eyes
+opened, the head was held erect more of the time. The young responded
+with "cries" when disturbed by handling, when stimulated by certain
+movements of the parent, or by movements of our hands near their heads,
+which suggested to them the possibility of being fed. Cries were evident
+but weak in the unhatched, pipped, egg, but soon after hatching
+increased in intensity, and beginning at six days of age were replaced,
+in part, by the characteristic "bill-snapping" of more mature birds.
+These cries of the young may serve, among other things, for recognition,
+inasmuch as they were given when the parent was inspecting the young.
+When the parent returned to the nest and covered the young, after having
+been flushed, it sometimes uttered a special note, "hut, hut, hut," much
+like the "cluck" note of the hen of the domestic chicken. The young
+responded to these notes with faint cries, in contrast to the loud cries
+signifying alarm and possibly hunger, which they gave when the parents
+were absent from the nest.
+
+The first definite evidence that the young were attempting to feed
+themselves was obtained when they were 23 days old. Frequently
+thereafter, fresh blood was found on their beaks and claws, but as late
+as the 34th day a parent was seen feeding them. That day, after being
+flushed, a parent returned to the nest at 7 p. m., and began tearing
+away parts of a cottontail which had previously been brought to the
+nest. Bones in the hind leg of the rabbit broke readily under pressure
+of the parent's bill, and the three young crowded in close, opening
+their bills widely and placing them around that of the parent. Of
+cottontails, the only parts consistently uneaten were the upper cheek
+teeth and the supporting maxillae and connecting palatal bridge.
+
+
+FOOD BROUGHT TO THE NEST
+
+In the 45 days that the young remained at the nest site in 1945,
+ninety-one individuals of 16 different species of animals were brought
+by the parent owls (table 2). Probably a few smaller animals, of which
+we saw no traces, were caught and eaten at night. In 1946, two
+additional kinds of birds were brought to the nest: 1 Baldpate (_Mareca
+americana_) and 1 Pied-billed Grebe (_Podilymbus podiceps_). The large
+number of Rock Doves in the list can be explained by their abundance on
+the buildings on the University campus, including the Museum building
+where some were nesting as close as 100 feet to the owl nest. When the
+owls were less than a week old, only small birds and mammals were
+brought (young Rock Doves, Robins, Starlings, Grasshopper Sparrow,
+meadow mouse, and Norway rat). The first rabbit was brought when the
+owls were eight days old.
+
+
+TABLE 2. Number of food items brought to the nest by the Great Horned
+Owls in 1945
+
+    Birds
+
+    Rock Dove (_Columba livia_)                       32
+    Robin (_Turdus migratorius_)                       6
+    Starling (_Sturnus vulgaris_)                      4
+    Mourning Dove (_Zenaidura macroura_)              10
+    Meadowlark (_Sturnella sp._)                       3
+    Red-wing (_Agelaius phoeniceus_)                   1
+    Bronzed Grackle (_Quiscalus versicolor_)           1
+    Mockingbird (_Mimus polyglottos_)                  1
+    Brown Thrasher (_Toxostoma rufum_)                 1
+    Grasshopper Sparrow (_Ammodramus savannarum_)      1
+    Coot (_Fulica americana_)                          3
+    Sora (_Porzana carolina_)                          1
+    Blue-winged Teal (_Anas discors_)                  1
+
+    Mammals
+
+    _Sylvilagus floridanus_                           19
+    _Rattus norvegicus_                                6
+    _Microtus ochrogaster_                             1
+
+After the 28th day, only 18 food items, or slightly less than 20 per
+cent of the total number, were brought to the nest. These last 18 food
+items brought after the owls were 4 weeks old were no larger or bulkier
+than those brought in the previous 20 days. The beginning of this period
+of reduced amount of food corresponds to the beginning of the second
+phase of growth characterized by marked fluctuations in weight.
+
+Fox squirrels (_Sciurus niger_) are abundant on the University campus,
+yet there were no remains of this mammal at the nest. This may be
+explained by the fact that fox squirrels are principally diurnal and
+Great Horned Owls feed principally at night. Yet in early February,
+1946, when the owls were preparing the nest, they frequently flew on and
+off the nest in the early twilight of evening while one or two fox
+squirrels fed in the periphery of trees not more than 25 feet away. Yet
+the owls flew off to the west and left this source of food unmolested.
+
+Whether both owls regularly attended the young we do not know, for the
+adults were not distinctively marked. On March 17, 1945, when weighing
+the young, one parent bird started to return to the nest but was
+frightened away by the observer who at the same time noted the other
+parent perched in an adjacent tree. This was the first time two adults
+were seen at the same time near the nest. In 1946, two adult owls
+(presumably both were parents) were within sight at one time when the
+young owl first sailed forth and landed in a wooded area some 100 yards
+away.
+
+
+SUMMARY
+
+Great Horned Owls (_Bubo virginianus virginianus_) have employed as nest
+sites the protruding shelves of the stone wall of the Museum of Natural
+History at the University of Kansas for several years. In 1945, daily
+observations were made on one such nest and its three young, and in 1946
+irregular observations were made on another such nest and the one young
+owl. The incubation time for the three owls, hatched in 1945, was 35
+days for two of the young and 34 days for the third; for the one owl
+hatched in 1946, the incubation time was at least 33 days. Two owls were
+consistently smaller; when these smaller two left the nest they were,
+respectively, 21 and 17 per cent lighter than the other two. The smaller
+two were judged to be males because adult males in Kansas average
+smaller by 21 per cent than adult females.
+
+Growth of the nestling young is divisible into (1) a period of rapid
+increase in weight during the first 25 to 28 days, and (2) a subsequent
+period marked by gains and losses in weight. The fluctuations in this
+latter period are correlated with a reduction in food brought to the
+nest by the parent birds and with the development of habits of flight.
+This second period may be considered to be a period of "weaning." By
+the forty-fifth day, the young owls are able to fly short distances and
+thus are able to leave the site of the nest permanently. At this time
+they are about three-fourths grown.
+
+Ninety-one individuals of 16 species of birds and mammals made up the
+food items brought to the nest in 1945. Two factors seem to be concerned
+in the acquisition of prey: (1) its availability and (2) appropriate
+size of the prey.
+
+
+
+
+LITERATURE CITED
+
+BAUMGARTNER, F. M.
+
+1938. Courtship and nesting of the Great Horned Owls. Wilson Bull.,
+50:274-285.
+
+
+BENT, A. C.
+
+1938. Life histories of North American birds of prey (Part 2), Orders
+Falconiformes and Strigiformes. U. S. Nat. Mus., Bull. 170, viii + 482
+pp.
+
+
+FITCH, H. S., SWENSON, F., and TILLOTSON, D. F.
+
+1946. Behavior and food habits of the Red-tailed Hawk. Condor,
+48:205-237.
+
+
+HUGGINS, S. E.
+
+1940. Relative growth in the House Wren. Growth, 4:225-236.
+
+
+RIDDLE, O., CHARLES, D. R., and CAUTHEN, G. E.
+
+1932. Relative growth rates in large and small races of pigeons. Proc.
+Soc. Exp. Biol. and Med., 29:1216-1220.
+
+
+STONER, D.
+
+1935. Temperature and growth studies on the Barn Swallow. Auk,
+52:400-407.
+
+1945. Temperature and growth studies of the Northern Cliff Swallow. Auk,
+62:207-216.
+
+
+SUMNER, E. L., JR.
+
+1929a. Notes on the growth and behavior of young Golden Eagles. Auk,
+46:161-169.
+
+1929b. Comparative studies in the growth of young raptores. Condor,
+31:85-111.
+
+1933. The growth of some young raptorial birds. Univ. California Publ.
+Zooel., 40:277-307.
+
+1934. The behavior of some young raptorial birds. Univ. California Publ.
+Zooel., 40:331-361.
+
+
+[Illustration: FIG. 3. Young Great Horned Owls in nest. Two owls are 7,
+12, 18, 32, and 36 days of age, respectively; the third owl is about 2
+days younger in each instance.]
+
+[Illustration: FIG. 4. Young Great Horned Owl hatched in 1946. The two
+lower pictures show the developing facial mask. Photographs by Joao
+Moojen.]
+
+[Illustration: FIG. 5. Young Great Horned Owl hatched in 1946. Upper
+row: Ventral views showing scar of yolk sac and ventral side of wing.
+Middle row: Ventral (left) and dorsal view of wing at 21 days. Bottom
+row: Ventral (left) and dorsal view of wing at 47 days. Photographs by
+Joao Moojen.]
+
+
+21-6958
+
+
+
+
+
+End of the Project Gutenberg EBook of The Postnatal Development of Two
+Broods of Great Horned Owls, by Donald F. Hoffmeister and Henry W. Setzer
+
+*** END OF THIS PROJECT GUTENBERG EBOOK THE POSTNATAL DEVELOPMENT OF ***
+
+***** This file should be named 35118.txt or 35118.zip *****
+This and all associated files of various formats will be found in:
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