Forest Fragmentation Affects the Behavioral Response of American Redstarts to the Threat of Cowbird Parasitism

Defensive behavior of American Redstarts (Setophaga ruticilla) in response to models of a female Brown-headed Cowbird (Molothrus ater) and a control (Spizella passerina, Passerella iliaca, or Melospiza melodia) placed near their nests was investigated in fragmented and contiguous boreal forest of western Canada. Female redstarts usually discovered and responded first to models but males also participated in nest defense. Based upon several behavioral measures, which included distraction displays, close passes or strihng the model, and alarm "chip" or "buzz" vocalizations, female redstarts reacted more intensely or aggressively to the cowbird model in fragmented compared with contiguous forested landscapes. We interpret these differences as evidence that redstarts are more familiar with cowbirds in fragmented landscapes because cowbirds are more common there and pose a greater threat to this host's reproductive success. Our findings have implications for interpretations of the demography of redstart populations and the fixation of antiparasite behavioral responses within populations. We suggest that tests using models provide a viable means of evaluating the influence of landscape matrix on the severity of cowbird exposure to host populations.