and
yet the four nests built by this pair were placed in them. This is the
only instance of seeming preference.

_Building_

Nestbuilding by Bell Vireos can be best discussed in terms of the
phases of construction described for the Red-eyed Vireo, Lawrence
(1953:57), which are: (1) construction of the suspension apparatus,
(2) construction of the bag, (3) lining of the bag and smoothing and
polishing of the exterior, and (4) adornment of the exterior. Red-eyes
(Lawrence, 1953:59) may continue adornment far into the period of
incubation. Both the male and female Bell Vireo have been observed to
add spider egg sacs and other silk to the exterior of the nest as late
as the sixth day of incubation.

Nice (1929:16) recorded only the female Bell Vireo building, but she
did recall, from previous studies, having seen males aiding somewhat.
Pitelka and Koestner (1942:102) wrongly concluded that the female Bell
Vireo builds unaided, but Hensley (1950:243) observed that both sexes
participated in nestbuilding, and Mumford (1952:229) reported two
instances of building by both adults. His description of the
activities viewed in mid-May suggest that they were of the
transitional period between the first and second phases. On the second
occasion he recorded both adults building during the second phase.
Since no details accompany this second observation I assume that it
pertained to activity not necessarily typical of this phase of
construction. Whereas both sexes of the Bell Vireo cooperate in
building the nest, only the female Red-eyed Vireo builds according to
Lawrence (1953:56). But Common (1934:242) saw both Red-eyed Vireos
building a nest.

The suspension apparatus is constructed by only the male on the first
day. He punctuates each trip to the nest with song. The single song
phrase is given from three to eight times when the male, carrying
nesting material in his bill, arrives in the tree. Typically, he
alights on several perches within the nest tree before flying to the
nest. He often interrupts his work with several songs; when he has
finished adding a load of material he sings from several perches
within the nest tree before departing. The male periodically stops
building to court the female.

In eight hours (494 minutes) of observing the first phase of
construction at five different nests, I saw the female come to the
nest 28 times; the male made 95 trips. The female came alone only
once, and brought nesting mater