Not so colourful after all: eggshell pigments constrain avian eggshell colour space

Birds' eggshells are renowned for their striking colours and varied patterns. Although often considered exceptionally diverse, we report that avian eggshell coloration, sampled here across the full phylogenetic diversity of birds, occupies only 0.08–0.10% of the avian perceivable colour space. The concentrations of the two known tetrapyrrole eggshell pigments (protoporphyrin and biliverdin) are generally poor predictors of colour, both intra- and interspecifically. Here, we show that the constrained diversity of eggshell coloration can be accurately predicted by colour mixing models based on the relative contribution of both pigments and we demonstrate that the models' predictions can be improved by accounting for the reflectance of the eggshell's calcium carbonate matrix. The establishment of these proximate links between pigmentation and colour will enable future tests of hypotheses on the functions of perceived avian eggshell colours that depend on eggshell chemistry. More generally,...鸟类蛋壳以其绚丽夺目的色彩与丰富多样的斑纹而闻名于世。尽管此前普遍认为鸟类蛋壳色彩极为多样，但本研究通过覆盖鸟类全部系统发育多样性的采样发现，鸟类蛋壳色彩仅占据鸟类可感知色彩空间的0.08%~0.10%。两种已知的四吡咯类蛋壳色素——原卟啉（protoporphyrin）与胆绿素（biliverdin）的浓度，无论在种内还是种间水平上，通常都无法很好地预测蛋壳色彩。本研究表明，基于两种色素的相对贡献量构建的色彩混合模型，可以准确预测蛋壳色彩的受限多样性；进一步研究发现，若将蛋壳碳酸钙基质的反射率纳入考量，模型的预测精度还能得到提升。这些色素沉积与色彩之间的近因关联的建立，将为未来检验基于蛋壳化学的鸟类蛋壳色彩功能假说提供可能。更广泛而言，