Heterospecifics removing eggs: brood parasites or predators of nest content?

Many bird species adjust their nest defense behavior according to the type and magnitude of perceived threats. Hosts of brood parasites – species that rely on another species effort to raise its offspring – are particularly suitable for studying such discrimination because they must defend their nests against both parasites and predators. Previous studies have typically compared host responses to brood parasites, which pose no direct threat to host adults, with responses to predators threaten, which threaten both eggs and adults. We tested whether yellow warblers (Setophaga petechia) discriminate between two heterospecific intruders that provide a similar functional cue –namely, the removal of an egg from the nest – by comparing their responses to the brood parasitic brown-headed cowbird (Molothrus ater) and predatory gray catbird (Dumetella carolinensis), which targets only nest contents and does not threaten adult birds. We predicted that cowbird and catbird both would elicit similar responses, and tested it through comparisons between the main behavioral responses in warbler defense repertoire. Our results confirmed a parasite-specific response to cowbirds, including seet calls and nest-protection behavior that differed from those elicited by catbirds. Given that cowbirds and catbirds provide similar informational cues at the nest (i.e., egg removal), it remains unclear how hosts differentially associate these two types of threats. We suggest that including nest predators that are innocuous to host adults as comparative species may help reveal the associative mechanisms underlying such refined discriminatory abilities.