The Postnatal Development of Two Broods of Great Horned Owls.

by Donald F. Hoffmeister and Henry W. Setzer.

Opportunity regularly to observe at the nest the development of young Great Horned Owls, _Bubo virginia.n.u.s_ (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 a.s.sumed 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 lat.i.tude 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 a.s.sumed 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, a.s.suming 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 part.i.tions 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 s.e.xual 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 p.r.o.nounced secondary s.e.xual 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 princ.i.p.al 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 coordinated 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 virginia.n.u.s pacificus_ Ca.s.sin.

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, Gra.s.shopper 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 (_t.u.r.dus 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 Gra.s.shopper Sparrow (_Ammodramus savannarum_) 1 Coot (_Fulica americana_) 3 Sora (_Porzana carolina_) 1 Blue-winged Teal (_Anas discors_) 1

Mammals

_Sylvilagus florida.n.u.s_ 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 princ.i.p.ally diurnal and Great Horned Owls feed princ.i.p.ally 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.