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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 João
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
João Moojen.]


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