Data from: Carotenoid-based bill coloration functions as a social, not sexual, signal in songbirds (Aves: Passeriformes)

Many animals use coloration to communicate with other individuals. While the signalling role of avian plumage colour is relatively well studied, there has been much less research on coloration in avian bare parts. However, bare parts could be highly informative signals as they can show rapid changes in coloration. We measured bill colour (a ubiquitous bare part) in over 1600 passerine species and tested whether interspecific variation in carotenoid-based coloration is consistent with signalling to potential mates or signalling to potential rivals in a competitive context. Our results suggest that carotenoid bill coloration primarily evolved as a signal of dominance, as this type of coloration is more common in species that live in social groups in the non-breeding season, and species that nest in colonies; two socio-ecological conditions that promote frequent agonistic interactions with numerous and/or unfamiliar individuals. Additionally, our study suggests that carotenoid bill coloration is independent of the intensity of past sexual selection, as it is not related to either sexual dichromatism or sexual size dimorphism. These results pose a significant challenge to the conventional view that carotenoid-based avian coloration has evolved as a developmentally costly, condition-dependent sexual signal. We also suggest that bare part ornamentation may often signal different information than plumage ornaments.

诸多动物均会借助体色与其他同类开展交流。尽管学界对鸟类羽色的信号功能已有较为充分的研究，但针对鸟类裸露部位体色的相关探究却相对匮乏。然而，裸露部位可作为极具信息量的信号载体，因其体色能够快速发生变化。本研究对1600余种雀形目（Passeriformes）鸟类的喙色（一种普遍存在的裸露部位）进行了测量，并检验了基于类胡萝卜素（carotenoid）的体色的种间差异，是否与向潜在配偶传递信号，或是在竞争场景下向潜在竞争者传递信号的假说相符。研究结果表明，基于类胡萝卜素的喙色主要是作为支配地位的信号演化而来：这类体色在非繁殖期营群居生活的物种，以及集群筑巢的物种中更为常见——这两种社会生态条件均会促进个体与众多乃至陌生同类之间频繁发生对抗性互动。此外，本研究发现基于类胡萝卜素的喙色与过往性选择的强度无关，因为其与性二色性（sexual dichromatism）或性大小二态性（sexual size dimorphism）均无关联。上述结果对传统观点构成了重大挑战：传统观点认为，基于类胡萝卜素的鸟类体色是一种发育成本高昂、且依赖个体状态的性选择信号。本研究同时指出，裸露部位装饰所传递的信息往往与羽饰存在差异。