The Parasitic Behavior of the Bronzed Cowbird in South Texas

Abstract

Aspects of the breeding ecology of the Bronzed Cowbird (Molothrus aeneus) were studied from early April to mid-July or August 1980 to 1982 at the Santa Ana National Wildlife Refuge in southern Texas. Female Bronzed Cowbirds, similar to Brown-headed (M. ater) and Shiny (M. bonariensis) cowbirds, are generalists in their egg laying. Of five common host species on the refuge, all were equally and usually repeatedly parasitized. Of these, only the Green Jay (Cyanocorax yncas) and Longbilled Thrasher (Toxostoma longirostre) are larger than the cowbirds in body size and are better host choices based on their relative success as fosterers. Experimental parasitism of Couch's Kingbirds (Tyrannus melancholicus), Curve-billed Thrashers (Toxostoma curvirostre), and Great-tailed Grackles (Quiscalus mexicanus) showed that they eject cowbird eggs from the nest. Female Bronzed Cowbirds, like Shiny Cowbirds, pierce both host eggs and previously laid cowbird eggs. Approximately twice as many host eggs were pierced compared to cowbird eggs. In some parasitized nests, more than one cowbird laid eggs and final host clutch size was greatly reduced. The vocal reaction of host species to cowbird intrusion probably provided clues on nest location to nest-searching female cowbirds who were drawn to such stimuli. Like the other four parasitic cowbird species, Bronzed Cowbird nestlings possess no apparent special adaptation for brood parasitism. Their vocal nature, however, appears to give them a competitive edge against less vocal host nestmates.