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 | | | | | | | | | |
_Chee_ | | | | x | | | | | | |
Begging | x | | |