Within-Season Breeding Dispersal in Prairie Warblers and Other Passerines

We examine the hypothesis that avoidance of predation on the nest can select for within-season breeding dispersal, as it does for between-season breeding dispersal. Only five studies have adequately tested this hypothesis against a null hypothesis, e.g., that dispersal is more likely to occur following a failure than following a successful nesting attempt, and all support at least one of the predictions of the predation-avoidance hypothesis. We also find support for the hypothesis in Prairie Warblers (Dendroica discolor): late-cycle losses resulted in a greater probability of dispersal. Two alternative hypotheses may explain within-season breeding dispersal: dispersal to permit more rapid renesting and dispersal to avoid depleted resources. Neither is supported in Prairie Warblers. Only one of the five earlier studies has tested either alternative. Although there is a clear need for further testing of all of these hypotheses the rather limited existing data do suggest that predation-avoidance is important in establishing within-season patterns of breeding dispersal.