Host Resource Partitioning Among Sympatric Molothrine Generalist Brood Parasites

Abstract

We examine host partitioning among sympatric brood parasites in southeastern Arizona and South Florida. In Florida, range expansion of three cowbirds (Molothrus bonariensis shiny cowbird, M. ater brown-headed cowbird, and M. aeneus bronzed cowbird) has brought them together in a region where host species have never been parasitized before. Whereas in southeastern Arizona, bronzed cowbird and brown-headed cowbird have been in sympatry longer. That sympatry and the larger size separation between the smallest race of the brown-headed cowbird (M. a. obscurus) and bronzed cowbird are correlated with the larger cowbird primarily parastizing larger host species. Furthermore, morphometric analysis shows that brown-headed cowbirds are significantly smaller in sympatry than allopatry, suggesting that the size variation across their range is not only a function of clinal variation but driven, perhaps, by long-term interspecific competitive interactions for nests to parasitize (i.e., character displacement model). Interspecific competition in southeastern Arizona for nests to parasitize has led to some partitioning of the host resources. In South Florida, recent contact between three parasitic species suggests a different outcome. In South Florida, the shiny cowbird was recorded mainly from coastal areas, whereas brown-headed cowbirds were found in coastal and inland areas leading to a degree of habitat partitioning. In Florida, brown-headed cowbirds parasitized species ranging from 6 to 100 g, with the majority being smaller species (20 g or less). There are far fewer breeding records for shiny cowbirds, but the similarities in size and species use outside of Florida suggest host overlap with the brown-headed. The longer breeding season of the shiny cowbird suggests that, while there will be host overlap with the brown-headed, a temporal separation may occur. The larger bronzed cowbird parasitized the larger hosts in the community, e.g., red-winged blackbird and spot-breasted oriole.