Development of the Nestlings
Young are pinkish at hatching and devoid of visible natal down. Du Bois (in Wetherbee, 1957:380), inspected day-old nestlings by means of a magnifying glass and was unable to detect any down. Nolan (1960:236) also indicates that the young are naked at birth and that the "body color is between flesh and rufous except where folds of the straw yellow skin obscure the underlying colors." The Hutton Vireo (Vireo huttoni) is essentially naked at birth, save for sparse hairlike down on the head and back (Wetherbee, 1953:380). The Red-eyed Vireo, according to Lawrence (1953:67) is naked at birth save for a sparse covering of greyish natal down, on the head, shoulders, and back.
In the Bell Vireo the pterylae darken slightly on the second day and the color becomes more intense daily until the quills of the dorsal tracts, the wings, and the tail break from their sheaths on the sixth day. In Red-eyed Vireos the pterylae darken by the end of the first day and the quills break through the skin on the fifth day, erupting from the sheaths by the seventh day (Lawrence, 1953:67).
From the first day the young are able to squeak. Poking a young bird was sufficient to elicit this sound, phonetically a nasal peek. The only other vocalization noted throughout the nestling period was an abbreviated chee.
For the first three days tapping the nest or even movement of it caused by wind would elicit begging. By the fifth day at nest 2-a (1959) only vigorous agitation of the branch to which the nest was attached evoked any response. At this nest on June 16, 1959, one young begged while the other cowered. Cowering is correlated with opening of the eyes, as the young bird that begged had its eyes only partly open. Both young cowered on June 19, 1959. Table 9 summarizes the maturation of the nestling Bell Vireos.
Table 9. Maturation of Nestling Bell Vireos. The First Day That an Activity Was Observed Is Shown.
| Day of nestling life | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | |
| Eyes open | x | ||||||||||
| Feathers erupt | x | ||||||||||
| Sound: Squeak | x | ||||||||||
| Sound: Chee | x | ||||||||||
| Begging | x | ||||||||||
| Cowering | x | ||||||||||
| Head scratching and Preening | x | ||||||||||
| Hopping to rim of nest | x | ||||||||||
| Fledging | x[G] | ||||||||||
[G] This is the commonest fledging day.