Does Nest Defence Deter or Facilitate Cowbird Parasitism? A Test of the Nesting‐cue Hypothesis

The nesting‐cue hypothesis poses that avian brood parasites use nest‐defence responses directed toward them by hosts as cues to locate nests to parasitize. Hosts that respond more intensely to brood parasites should provide more cues about nest location than those hosts giving lower intensity responses. Thus, the nesting‐cue hypothesis predicts that within a species parasitized nest owners should respond more intensely than unparasitized nest owners to cowbirds perched near and away from nests. This assumes that hosts respond to cowbirds when they are encountered away from the nest and that host responses gradually increase in intensity as the cowbird nears the nest. The nesting‐cue hypothesis, its assumptions and prediction were tested using six host species of the brown‐headed cowbird ( Molothrus ater , Icterinae). We presented a female cowbird model at three distances from host nests and compared the responses elicited. All species responded to the cowbird at all distances, which supports the first assumption. Some of the rarely elicited responses (e.g. contacts) and the proximity of the host to the model varied significantly with distance, which suggests that cowbirds could use nest defence by the host as cues to the location of an active nest. However, parasitized nest owners did not respond more intensely than unparasitized nest owners to the cowbird positioned at any of the distances from the nest, which does not support the nesting‐cue hypothesis itself. Further considerations are discussed that suggest that nest defence is not likely to be used as a nest‐location cue.