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

Abstract 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. Significance statement Birds must defend their nests against multiple threats, but the optimal defense strategy depends on the type of intruder. Brood parasites, such as the brown-headed cowbird, harm host fitness primarily indirectly: they remove a host egg and, during subsequent visits, lay their own, with the resulting chick eventually monopolizing parental care and substantially reducing host offspring survival. Yellow warblers, an important cowbird host species, may prevent these costs by adopting a nest-defense strategy that may position them close to the parasites without exposing themselves to risk, as cowbirds pose no direct threat to adults. Nest predators, by contrast, may also prey on adults, making close mobbing potentially risky. However, predators that target only eggs or nestlings pose no danger to adults, and in principle, close mobbing may be similarly advantageous. Remarkably, yellow warblers respond differently to cowbirds and to nest-only predators, despite the similar functional cue of egg removal. How hosts form the cognitive association underlying such refined, threat-specific defense remains unknown, revealing a critical gap in our understanding of avian-decision making and threat discrimination.