How to Make a Mimic? Brood Parasitic Striped Cuckoo Eggs Match Host Shell Color but Not Pigment Concentrations

Hosts of avian brood parasites often use visual cues to reject foreign eggs, and several lineages of brood parasites have evolved mimetic eggshell coloration and patterning to circumvent host recognition. What is the mechanism of parasitic egg color mimicry at the chemical level? Mimetic egg coloration by Common Cuckoos Cuculus canorus is achieved by depositing similar concentrations of colorful pigments into their shells as their hosts. The mechanism of parasitic egg color mimicry at the chemical level in other lineages of brood parasites remains unexplored. Here we report on the chemical basis of egg color mimicry in an evolutionarily independent, and poorly studied, host-parasite system: the Neotropical Striped Cuckoo Tapera naevia and one of its hosts, the Rufous-and-white Wren Thryophilus rufalbus. In most of South America, Striped Cuckoos lay white eggs that are identical to those of local host species. In Central America, however, Striped Cuckoos lay blue eggs that match those of the Rufous-and-white Wren, suggesting that blue egg color in these cuckoo populations is an adaptation to mimic host egg appearance. Here we confirm that Striped Cuckoo eggs are spectrally similar to those of their hosts and consistently contain the same major eggshell pigment, biliverdin. However, wren eggshells lacked protoporphyrin, which was present in the parasitic cuckoo eggshells. Furthermore, biliverdin concentrations were significantly lower in cuckoo eggshells than in host eggshells. Similarity of host-parasite eggshell appearance, therefore, need not always be paralleled by a quantitative chemical match to generate effective visual mimicry in birds.