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.
Fig. 2. Morning and afternoon weights of two Great Horned Owls. Note that in the morning the owls weigh more than in the afternoon.
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 coördinated 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 | ...... |