anism of pair-formation in the Bell Vireo is not
known. My experience has been to find a male one day and then one or
two days later to discover that it has a mate. Lawrence (1953:53),
tells of a male Red-eyed Vireo singling out a female from a flock of
migrants passing through his territory and violently driving her to
the ground. Shortly after this attack the pair was seen searching for
a nest site. But such an incident has not been reported for other
vireos, nor have I witnessed such behavior myself.

Early courtship activities of the Bell Vireo are characteristically
violent affairs, with the male directing strong aggressive attacks
toward the female. Rapid, looping flights through the thickets occur,
the female leading the male. Occasionally he deliberately collides
with her in mid-air, but the pair quickly separate. This violent
sexual chasing is manifest prior to the inception of nestbuilding.
With commencement of this activity, sexual chases through the
territory subside.

Absence of sexual dimorphism in the Bell Vireo obviously suggests that
behavioral criteria are used by the birds in sex-recognition. The lack
of aggression by the female upon initial aggression by the male is an
essential component of recognition of sex; she is clearly subordinate.
Such subordination is also the significant feature of continued
sex-recognition. Courtship display by a resident male, directed toward
a stuffed male and a wounded male which sat motionless, supports the
contention that a subordinate or submissive attitude of the female is
a key factor in sex-determination.

Nestbuilding and courtship are intimately associated in this species.
The male constructs the suspension apparatus of the nest, the
completion of which coincides with the assumption of nestbuilding
activity by the female. Roles of the sexes in nestbuilding are
described in the section on nestbuilding. The male frequently
interrupts construction to court the female. This, in combination with
perpetual song as he works, serves to strengthen the pair-bond and
stimulate nestbuilding tendencies of the female.

It is doubtful that any attempts at copulation are successful up to
this time. The female is singularly unresponsive to the advances of
the male; a female retreats before most violent attacks and is
seemingly oblivious to less vigorous behavior. After the female
assumes the responsibility of building, the tempo of courtship
activities increases.

The fem