Factors influencing the incidence of cowbird parasitism of least Bell's vireos

Least Bell's vireo (Vireo bellii pusillus) is a federally endangered subspecies of Bell's vireo subject to high levels of brood parasitism by brown-headed cowbirds (Molothrus ater). Brood parasitism greatly reduces the reproductive success of the vireo. We examined the relationship of vegetation structure surrounding nests and of activity near the nest to the incidence of brood parasitism by brown-headed cowbirds of least Bell's vireos. We examined vegetation structure at 3 spatial scales: microhabitat (0-1 m from a nest), mesohabitat (1-11.3 m from a nest), and macrohabitat (greater than 11.3 m from a nest). Nests with high microhabitat cover and mesohabitat cover within 5 m of the nest had a lower incidence of parasitism than those with low cover at these scales. Unparasitized nests had fewer trees greater than 8-cm diameter at breast height (dbh) within 11.3 m, and they had less canopy cover within 5 m than parasitized nests. Cowbirds parasitized nests farther from the edge of the riparian habitat more often than nests near the edge. Activity near the nest did not differ significantly between parasitized and unparasitized nests. We suggest that microhabitat cover is the most important habitat feature influencing the incidence of brood parasitism of least Bell's vireos, and we conclude that cover near the nest reduces the chance that a cowbird will observe nesting activity. We suggest that habitat management for improved breeding success of least Bell's vireos focus on increasing the density of understory vegetation.