Bell's Vireo

Vireo bellii Audubon

The various races of Bell's vireo have long been known to be frequent victims of the brown-headed cowbird, a situation that is even more strongly emphasized by the rapid accumulation of additional instances. In 1963 there were some 82 actual instances compiled; by 1975 this has grown by more than 50 percent. The Western Foundation alone has in its collections some 46 parasitized sets of eggs (23.3 percent) out of a total of 197 sets. By subspecies, the figures are: V. bellii bellii and V. bellii medius, 58 sets, 17 (29.3 percent) parasitized; V. bellii arizonae, 13 sets, 5 (38.4 percent) parasitized; V. bellii pusillus, 120 sets, 24 (20.0 percent) parasitized.

There is a possibility that the high incidence of parasitism on this bird in southern California, coupled with an accelerated rate of destruction of its habitat may be causing it locally to diminish critically in numbers. Similarly, in the central valley of central and northern California, Remsen and Gaines (1973:916) considered that this vireo, as well as the yellow warbler and the yellowthroat, were nearly extinct, “probably due to cowbird parasitism.” The excessive victimization of Bell's vireo is also noted in southern Texas, where, in 1971, 23 nests were reported and 17 of these contained cowbird eggs or young (Webster, 1971:877).

YELLOW-THROATED VIREO