Part 2

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.

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