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THE POSTNATAL DEVELOPMENT OF TWO BROODS OF GREAT HORNED OWLS

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

The Postnatal Development of Two Broods of Great Horned Owls

DONALD F. HOFFMEISTER AND HENRY W. SETZER

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 .

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 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 that "the horned owl incubates very closely," for a strong stimulus was necessary to keep the owl from covering the eggs.

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 was weighed on the first day of life and on most subsequent days. The other two owls 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.

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 , increase in weight rapidly for about the first three weeks and then more gradually. Sumner's 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 , 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 , but after the fourth week there is a rapid increase in weight. Graphs that Sumner 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 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 in the Cliff Swallow. Huggins' 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 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 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 ; presumably they were gorged with food early in the morning.

TABLE 1.--Changes with age in certain parts of a young Great Horned Owl hatched in 1946.

Remnants of the yolk stalk were clearly evident at seven days of age in the owl hatched in 1946 and were still present when the owl was last examined 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 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 . 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 and 1 Pied-billed Grebe . 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 . 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 32 Robin 6 Starling 4 Mourning Dove 10 Meadowlark 3 Red-wing 1 Bronzed Grackle 1 Mockingbird 1 Brown Thrasher 1 Grasshopper Sparrow 1 Coot 3 Sora 1 Blue-winged Teal 1

Mammals

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 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 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 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.

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: its availability and appropriate size of the prey.

LITERATURE CITED

BAUMGARTNER, F. M.

BENT, A. C.

FITCH, H. S., SWENSON, F., and TILLOTSON, D. F.

HUGGINS, S. E.

RIDDLE, O., CHARLES, D. R., and CAUTHEN, G. E.

STONER, D.

SUMNER, E. L., JR.

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