per nest | .94 | .54
--------------------------------------+-------+----------

The percentage of cowbird eggs hatched in relation to the number laid
is relatively low. For instance, Mumford (1952:231) has only one
record of a young cowbird successfully raised by a Bell Vireo. The
data available in Bent (1950:260-261) also indicate that the
percentage of cowbird eggs hatched is small. The Bell Vireo is less
tolerant of cowbird parasitism than are many of the species so
victimized, but is not so intolerant as the Robin, Catbird, and the
Yellow-breasted Chat (Friedmann, 1929:193).

SUMMARY

1. The behavior of a small population of Bell Vireos was studied in
the spring and summer of 1959 and again in 1960 in Douglas County,
Kansas, and results are compared with previous studies elsewhere.

2. The Bell Vireo sings more often daily and throughout the nesting
season than do the majority of its avian nesting associates. Six types
of vocalizations are readily distinguishable in the field: primary
song, courtship song, distress call, alarm note, specialized male call
note or _zip_, and the generalized call note or _chee_.

3. Territories are established in early May and occupied throughout
the breeding season and post-breeding season. The average size of the
territories in 1960 was 1.25 acres. Shifting of territorial boundaries
occasionally occurs after nesting attempts.

4. Territory is maintained primarily by song, but at least five
aggressive displays are manifest in the early phases of territorial
establishment. These include: (a) vocal threat, (b) head-forward
threat, (c) wing-flicking and sub-maximal tail-fanning, (d) ruffling
and maximum tail-fanning, and (e) supplanting attack.

5. The precise mechanism of pair-formation in the Bell Vireo is not
known. Early courtship activities are characteristically violent
affairs. Absence of sexual dimorphism suggests that behavioral
criteria are used by the birds in sex-recognition; the male is
dominant and the female is subordinate.

6. The principal displays associated with courtship include: greeting
ceremonies, "pouncing," "leap-flutter," pre- and post-copulatory
displays, and the posture, copulation. The marked similarity between
elements of courtship display and aggressive display suggests common
origin or the derivation of one from the other.

7. The nest-site probably is selected by the female. Nests are
suspended from lateral or terminal forks abou