10. Egglaying begins on the first or second day after the nest is completed. The eggs are deposited early in the morning. The average clutch-size of the Bell Vireo in Kansas is 3.39 eggs.
11. Both sexes sit on the eggs, but only the female truly incubates because the male lacks a brood patch. Incubation lasts fourteen days.
12. The Bell Vireo is double-brooded in "good" years.
13. Nesting failure resulted from severe weather, predation, parasitism by cowbirds, and human interference. Behavior that contributes to nesting failure is selection of an unfavorable nest-site, singing on and near the nest, and the tendency to flush from the nest in view of potential enemies.
LITERATURE CITED
American Ornithologists' Union
1957. Check-list of North American birds. Fifth ed. Baltimore, The Lord Baltimore Press, The American Ornithologists' Union, iv + 691 pp.
Andrew, R. J.