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 about 2 feet 3 inches high in small trees and shrubs averaging 11 feet 2 inches in height.
8. Nestbuilding is intimately associated with courtship and is a responsibility of both sexes. The male builds the suspension apparatus and the female constructs and lines the bag. Both sexes participate in adorning the exterior. Construction lasts from four and one-half to five days.
9. The nest is compact, pendant, and composed of strips of bark and strands of grasses that are interwoven and tightly bound with animal silk. Nests built in May are bulkier than those constructed later in the season.