Breeding behavior of the Plumbeous Vireo in New Mexico

The Plumbeus Vireo (Vireo plumbeus) is a poorly studied songbird species th at breeds within the interior western United States and Mexico. We studied the breeding behavior of Plumbeous Vireos within pinylon-juniper habitats i n northeastern New Mexico in 1996 and 1997. Over both years we located and monitored 40 nests and conducted behavioral observations throughout the nes ting cycle. From these observations we described and compared basic behavio ral traits of male and female vireos during their nest-building egg-laying, incubation, and nestling stages, and their response to potential predators near the nest. Male vireos displayed potential nest sites: to females, but the females selected the site to be used and built most of the nest. Vireo s often did not initiate egg-laying until 2-3 d after nest completion. The female tended to incubate eggs and brood nestlings more than the male, but both parents contributed equally in provisioning the young. Brown-headed Co wbirds (Molothrus ater) parasitized 77% of nests in 1906, 63% in 1997, and parasitism was the primary cause of nest failure in this population. The co nspicuousness of vireos around the nest, particularly during the nest-build ing stage, may contribute to high parasitism levels. Vireo aggression near die nest may also act as a cue to help cowbirds locate nests.