The Postnatal Development of Two Broods of Great Horned Owls (bubo Virginianus)

Part 1

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THE POSTNATAL DEVELOPMENT OF TWO BROODS OF GREAT HORNED OWLS

(Bubo virginianus)

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)

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.

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.

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