Data from: The perceptual and chemical basis of egg discrimination in communally nesting Greater Anis (Crotophaga major)

The eggshells of communally breeding Greater Anis (Crotophaga major) consist of a blue-green pigmented calcite matrix overlaid by a chalky white layer of vaterite, both of which are polymorphs of calcium carbonate. The white vaterite layer is intact in freshly laid eggs and may function in protecting the eggs from mechanical damage, but it also abrades during incubation to reveal the blue calcite shell underneath. Previous research has shown that this color change serves a visual signaling function: nesting Greater Anis can discriminate between eggs that are freshly laid and those that have already been incubated, which allows them to reject asynchronous eggs laid by extra-group parasites. Here we use avian visual modeling and pigment extraction to assess the perceptual and chemical bases of such egg recognition. We found that there was no overlap between the avian perceptual space occupied by ani eggshells with and without vaterite, and that vaterite lacked both of the pigments found in the eggshell’s calcite matrix, bililverdin and protoporphyrin. The visual contrast between the unpigmented vaterite and the blue-pigmented calcite appears to pre-date the evolution of the signaling function, since the related Guira Cuckoo (Guira guira), also a communal breeder, lays similarly structured and pigmented eggs but does not use the visual contrast as a signal to detect parasitism.

群居繁殖的大滑嘴犀鹃（Crotophaga major）的蛋壳由蓝绿色素方解石（calcite）基质与覆盖其上的垩白色球霰石（vaterite）层构成，二者均为碳酸钙的同质多象变体。新鲜产下的蛋中，白色球霰石层完整无缺，可发挥保护蛋壳免受机械损伤的作用，但在孵化过程中该层会逐渐磨损，露出下方的蓝色方解石蛋壳。既往研究表明，这种颜色变化具备视觉信号功能：筑巢的大滑嘴犀鹃能够区分新鲜产下的卵与已孵化过的卵，从而可以拒绝群外寄生者产下的异步卵。本研究采用鸟类视觉建模与色素提取技术，探究此类蛋壳识别行为的感知与化学基础。研究发现，带有球霰石层与不带球霰石层的犀鹃蛋壳在鸟类感知空间中不存在重叠，且球霰石中缺乏方解石基质所含的两种色素：胆绿素（biliverdin）与原卟啉（protoporphyrin）。无色素的球霰石与蓝色素方解石之间的视觉对比度，似乎早于该信号功能的演化——同为群居繁殖鸟类的近缘物种圭拉鹃（Guira guira）所产的蛋壳结构与色素组成均与之类似，但并未利用这种视觉对比度来检测寄生行为。