In its notice of Rulemaking Actions proposing the listing of both the Least Bell’s Vireo (Vireo bellii pusillus) and the Arizona Bell’s Vireo (V. b. arizonae) as endangered species, the U.S. Fish and Wildlife Service (1980) attributed the birds' decline in California both to loss of habitat and to parasitism by Brown-headed Cowbirds (Molothrus ater). As I recalled my casual observations of the Arizona Bell’s Vireo from 1976 to 1978, they did not indicate heavy parasitism by cowbirds.