Melanin is the most common pigment in avian plumage and has various functions, including signalling individual condition, social status, aiding in camouflage and strengthening feathers. To date, most research focusing on melanin‐based colours has focused on eumelanin black and pheomelanin rusty traits. The occurrence and function of grey melanin‐based coloration, however, remains largely unstudied. Using colour plates from a bird identification guide and a comparative phylogenetic approach, we analysed the occurrence and relative patch size of grey plumage regions in 239 passerine species belonging to 36 families occurring in the Western Palaearctic. We proposed four hypotheses to explain the evolution of grey plumage: (1) protection from visually oriented predators, (2) badges of social information, (3) sexually selected ornaments, and (4) visual enhancement of badges and ornamental colour patches via high between‐colour contrast. We tested these hypotheses by analysing the occurrence and relative size of grey patches in relation to ecological and life‐history traits and the occurrence of other colours. Moreover, we surveyed the distribution of grey over avian body regions and its adjacency to other plumage colours. Grey coloration was present in 97 species from 25 families (40.5% of all species and 69.4% of all families sampled, respectively). Our results support the idea that grey plumage evolved to provide cryptic coloration and/or as a sexual signal used in female mate choice. Grey colour occurrence was associated with less concealed nest structure and open habitats, and this effect was especially pronounced in species in which males incubate. In both sexes, grey was more abundant on the head and back regions, which are the most visible body parts during incubation. The occurrence of grey was related to a greater dichromatism and was slightly higher in males than in females, which suggests that it could have evolved in response to female mate choice. No evidence suggests that grey plumage enhances ornamental colours or acts as a competitive signal. The study provides the first insight into potential functions of grey plumage in birds. Further studies at the species level are needed to test our findings.

中文翻译：

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西部古北洋鸣鸟中黑色素灰色着色的发生和功能（Aves：Passeriformes）

黑色素是禽类羽毛中最常见的色素，具有多种功能，包括发信号通知个体状况，社会地位，帮助伪装和增强羽毛。迄今为止，大多数针对黑色素的颜色的研究都集中在黑色素和苯丙氨酸生锈的特征上。然而，基于灰色黑色素的着色的发生和功能仍未得到充分研究。使用鸟类鉴定指南中的色板和比较的系统发育方法，我们分析了在西古北洋中属于36个科的239种雀形目物种中灰色羽毛区域的发生和相对斑块大小。我们提出了四个假设来解释灰色羽毛的演变：（1）避免受到视觉导向的掠食者的侵害；（2）社交信息的徽章；（3）性选择的装饰品；（4）通过较高的色间对比来视觉增强徽章和装饰色块。我们通过分析与生态和生活史特征以及其他颜色的发生有关的灰色斑块的发生和相对大小，检验了这些假设。此外，我们调查了禽类身体区域的灰色分布及其与其他羽毛颜色的相邻性。灰色着色存在于25个科的97种中（分别占所有物种的40.5％和69.4％）。我们的结果支持这样的想法，即灰色羽毛进化为提供隐秘的颜色和/或作为女性伴侣选择中使用的性信号。灰色的出现与隐匿的巢结构和开放的栖息地有关，这种影响在雄性孵化的物种中尤为明显。在男女中，在孵化过程中最明显的身体部位是头部和背部，灰色更为丰富。灰色的发生与更大的二色性有关，并且男性比女性稍高，这表明它可能是根据雌性伴侣的选择而进化的。没有证据表明灰色的羽毛可以增强装饰色或充当竞争性信号。该研究为鸟类灰色羽毛的潜在功能提供了第一个见识。需要在物种水平上进行进一步研究以检验我们的发现。没有证据表明灰色的羽毛可以增强装饰色或充当竞争性信号。该研究为鸟类灰色羽毛的潜在功能提供了第一个见识。需要在物种水平上进行进一步研究以检验我们的发现。没有证据表明灰色的羽毛可以增强装饰色或充当竞争性信号。该研究为鸟类灰色羽毛的潜在功能提供了第一个见识。需要在物种水平上进行进一步研究以检验我们的发现。