Nest desertion and apparent nest protection behavior by Bell's Vireos in response to Cowbird parasitism

Bell's Vireos (Vireo bellii) deserted 51 % of nests parasitized by Brown-he aded Cowbirds (Molothrus ater) in central Missouri from 1996-1998. Desertin g vireo pairs fledged more host young within a season than pairs that accep ted cowbird eggs when only successful nests were considered; parasitized ac ceptor nests never fledged any vireo young. Vireo pairs that deserted did n ot always desert parasitized nests. We observed five encounters between fem ale cowbirds and Bell's Vireos at four nests. In one observation the female vireo used nest-protection behavior, which resulted in the cowbird egg app earing beneath the nest; this nest was not deserted. We found a total of ei ght nests where cowbird eggs remained on the ground below nests, five of wh ich were abandoned. Nest desertion appears to benefit Bell's Vireos by allo wing for unparasitized renests, but the stimuli eliciting nest desertion by Bell's Vireos remain in need of further study.