Coevolution and the diversification of nestling ornamentation in a species-rich avian radiation

Conspicuous juvenile phenotypes are puzzling to evolutionary biologists. Why should organisms vulnerable to predation boldly broadcast their presence? We reconstructed the evolutionary history of juvenile phenotypes across the estrildid finches (family Estrildidae), a radiation exhibiting unparalleled diversity in nestling ornamentation. Many are parasitized by Vidua finches whose offspring mimic host nestling phenotypes. We examined the role of brood parasitism, predation, sibling competition, and signaling environment in the diversification of nestling ornamentation. We found that parasitized lineages exhibit elevated rates of nestling ornamentation evolution compared to unparasitized lineages. Despite this, the extent to which nestlings were ornamented did not differ between parasitized and unparasitized lineages, contrasting with systems where coevolution proceeds at the egg stage and generates increased complexity in host traits. Species occupying denser habitats had increased ornamentation, suggesting a role for light environment in the evolution of begging displays. Nestling appearance showed a strong phylogenetic signal, helping to explain why successfully colonized hosts are often closely related to ancestral ones. Neither nest height nor clutch size (proxies for predation and sibling competition) predicted nestling ornamentation levels, and parasitism did not predict estrildid finch diversification rates. Overall, our results support a model of trait diversification in which hosts lead and parasites follow in the coevolutionary arms race.