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 |...... --------------------------+------+------+------+------+------+------+------