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 thisa broken egg was found beneath the nest and there were remnants of the egg in the nestincubation 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.
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.