Data from: The effect of colour-producing mechanisms on plumage sexual dichromatism in passerines and parrots

Sexual dichromatism (SD) often reflects intense sexual selection on males. It has been hypothesized that sexual selection should favour the elaboration of those male colours that honestly signal quality and that such colours should therefore show higher SD. Costliness of colours is expected to vary according to their production mechanism (pigment type, feather microstructure and combinations thereof). Carotenoid-based colours, due to their dietary origin and competing functions of carotenoid pigments, are the best documented costly colours, while endogenous pigments and structural colours are expected to be less costly. However, how SD varies with colour-producing mechanism has not been systematically addressed. Here, we test the link between SD and mechanisms of colour production across nearly all Australian passerines (n = 302) and parrots (n = 48), using reflectance spectrometry and visual models. We quantified SD [chromatic (hue and saturation) and achromatic (light to dark)] for individual plumage patches as well as for the whole plumage. We then compared whether these SD estimates vary for plumage produced by different mechanisms of colour production. Not unexpectedly, individual plumage patches were most sexually dichromatic when males and females have different mechanisms of colour production. In most cases however, both sexes share the same mechanism. Among these, red carotenoid-based plumage in passerines showed the highest SD, while in passerines as well as in parrots, melanin-based plumage and white plumage have the lowest levels of SD. Average SD within species is weakly predicted by the proportion of the plumage produced by different mechanisms. This effect is strongest for red carotenoids, but equally so for males and females. In contrast, effects of structural and pheomelanin-based colours are different between sexes, highlighting the links between high SD and the evolution of different mechanisms of colour production in males and females. While our analysis showed that colours based on red carotenoids have higher levels of SD, the differences were small, and most mechanisms of colour production were able to produce a similar range of SD. These results do not support the hypothesis that sexual selection should preferentially lead to the elaboration of some types of colours over others.性双色性（sexual dichromatism, SD）通常反映了对雄性的强烈性选择。已有假说提出，性选择会青睐那些能诚实地传递个体品质的雄性色彩的演化强化，因此这类色彩应表现出更高的SD。色彩产生的成本预期会随其产生机制（色素类型、羽毛微观结构及其组合）而有所差异。类胡萝卜素基着色因源于膳食且类胡萝卜素色素存在竞争性功能，是目前证据最充分的高成本着色；而内源性色素与结构色则被认为成本较低。然而，性别双色性如何随色彩产生机制变化尚未得到系统性研究。 本研究针对几乎所有澳大利亚雀形目鸟类（n=302）与鹦鹉类（n=48），利用反射光谱法与视觉模型，检验了SD与色彩产生机制之间的关联。我们对单个羽毛斑块以及整体羽毛的SD进行了量化，涵盖色度（色相与饱和度）与消色差（明暗程度）两个维度。随后我们比较了不同色彩产生机制所生成羽毛的SD估计值是否存在差异。 不出所料，当雌雄个体拥有不同的色彩产生机制时，单个羽毛斑块的性别双色性最为显著。但多数情况下，两性共享相同的色彩产生机制。在此类情形中，雀形目鸟类的红色类胡萝卜素基羽毛表现出最高的SD；而无论是雀形目还是鹦鹉类，黑色素基羽毛与白色羽毛的SD水平均最低。 物种内部的平均SD可通过不同机制生成的羽毛占比进行弱预测。这一效应在红色类胡萝卜素基着色中最为显著，且在雌雄两性中表现一致。与之相反，结构色与褐黑色素基着色的效应在两性间存在差异，这凸显了高SD与雌雄两性演化出不同色彩产生机制之间的关联。 尽管本研究的分析显示，基于红色类胡萝卜素的着色拥有更高的SD水平，但二者间的差异较小，且多数色彩产生机制所能产生的SD范围较为相似。上述结果并不支持“性选择会优先推动部分类型色彩的演化强化”这一假说。