FOREST FRAGMENTATION AFFECTS THE BEHAVIORAL-RESPONSE OF AMERICAN REDSTARTS TO THE THREAT OF COWBIRD PARASITISM

Defensive behavior of American Redstarts (Setophaga ruticilla) in resp onse 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 defe nse. Based upon several behavioral measures, which included distractio n displays, close passes or striking the model, and alarm ''chip'' or ''buzz'' vocalizations, female redstarts reacted more intensely or agg ressively to the cowbird model in fragmented compared with contiguous forested landscapes. We interpret these differences as evidence that r edstarts are more familiar with cowbirds in fragmented landscapes beca use cowbirds are more common there and pose a greater threat to this h ost's reproductive success. Our findings have implications for interpr etations 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 popul ations.