Two Subspecies of Warbling Vireo Differ in Their Responses to Cowbird Eggs

Using real cowbird eggs, we experimentally parasitized 41 nests of the Warbling Vireo (Vireo gilvus), three each in British Columbia and Colorado, five in Montana, and 30 in Manitoba, and recorded whether the cowbird eggs were accepted or rejected.Cowbird eggs were accepted at all nests tested in British Columbia and Colorado, but both acceptance and rejection were recorded in Montana.In Manitoba, all cowbird eggs were rejected (29 by puncture-ejection, one by desertion).The results suggest acceptance by a western subspecies of the Warbling Vireo, V. g. swainsonii, and rejection by the eastern subspecies, V. g. gilvus.The geographic variability in acceptance/rejection agrees with suggested taxonomic differences for the Warbling Vireo, i.e., that there are two species and that neither appears to vary in response to the presence of cowbird eggs in its nests.Species of birds that suffer reduced reproductive success when parasitized by Brown-headed Cowbirds (Jolothrus ater) should evolve strategies that reduce or eliminate the costs of parasitism, especially because cowbird eggs are distinguishable from the eggs of most host species.Adaptations for rejection of parasitic eggs from nests have evolved in some species, but most species accept cowbird eggs (Rothstein 1975(Rothstein , 1990)).Why only a few species reject cowbird eggs has been attributed to the duration of exposure to the selective pressure of cowbird parasitism (Rothstein 1975) or constraints on the ability of small hosts to eject cowbird eggs from the nest (Rohwer and Spaw 1988).Among the ejecter species, the larger ones grasp and remove cowbird eggs with their bills, whereas the smaller species puncture-eject them (Rohwer and Spaw 1988).Recently, Sealy (1996) determined experimentally that the Warbling Vireo (Vireo gilvus) removes cowbird eggs from its nests.At 15 g, it is the smallest species in North America known to do so (Sealy 1996).This species' responses to cowbird eggs also appear to vary across its geographic range.A literature survey of the frequency of parasitism in various populations of Warbling Vireos (table 2 of Sealy 1996) revealed that 0 to 11% of Warbling Vireo nests were parasitized in populations within and east of the Central Great Plains, whereas 50 to 70% of nests were parasitized in populations west of the Great Plains.Where experimental data are lacking, Friedmann et al. (1977) assumed that a species accepts cowbird eggs (i.e., is an accepter) if 20% or more of its nests are recorded parasitized.Under this definition, western populations of the Warbling Vireo would be classified as accepters, whereas central and eastern populations reject parasitism.Sealy's (1996) experiments on Warbling Vireos in Manitoba support the assessment of birds there as rejecters.

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