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.