Unpalatable birds are black: a re-analysis of Hugh Cott’s data on bird palatability

Abstract Many animals are aposematic, signalling their unprofitability via sounds, smells, or conspicuous colours. Although some birds are toxic, it remains unclear whether aposematism is broadly distributed in birds. Here, we re-analyse historical data compiled by Hugh Cott on bird palatability as assessed by humans and hornets. We use phylogenetic analyses to link palatability to plumage coloration and other covariates. Our analyses reveal that bird palatability has a moderate phylogenetic signal and that species with low palatability are more black. We also show that low palatability is associated with an animal-based diet and, paradoxically, with safer nests. However, accounting for the effects of these covariates in the model does not change the effects of colour. These results support Cott’s original hypothesis that birds can signal distastefulness through their colours. The convergence on black coloration might constitute a case of Müllerian mimicry. We hypothesize that black is a suitable aposematic colour because melanization is often associated with needs for detoxification. Our results suggest that aposematism in birds might be more widespread than assumed, but they need to be corroborated using other relevant predators of birds.