Pigment concentrations only partially predict avian eggshell colour mimicry in a polymorphic host-brood parasite system.

The precise mimicry of host eggshell colours and patterns by some obligate avian brood parasites provides a powerful study system for understanding co-evolutionary arms races. However, most attention has focused on host behaviour in response to mimicry, rather than the proximate mechanisms that give rise to mimetic eggshell colours and patterns. In Africa, the cuckoo finch produces a wide range of eggshell colours that largely match those of one of its hosts, the tawny-flanked prinia . Here, we use chemical analysis and avian visual modelling to determine how cuckoo finches and prinias produce this wide array of perceived eggshell colours. Although both species incorporate similar proportions of the two main egg pigments (biliverdin and protoporphyrin) into their eggshell matrix, the cuckoo finch deposits greater concentrations of pigments. Proportions of pigments predicted eggshell coloration only in bluer eggshells, whereas there was no statistical relationship between pigments and colours in browner eggshells. Possible explanations include that brown pigments are involved in maculation and/or strengthening the eggshell. Overall, variation in the deposition of eggshell pigments results in complex relationships between eggshell chemistry and avian-perceivable coloration, both within and across species in this system.