Data from: Differential visual ornamentation between brood-parasitic and parental cuckoos

The evolution of brood parasitism should affect adult phenotypic traits due to sexual selection as well as the parasite-host interactions, though it is rarely focused on. Sexual selection theory predicts extravagant secondary sexual characteristics in brood parasites while immature-like modest sexual characteristics in parental species. This is because juvenile-like immature traits can attract mates by exploiting parental care for young (i.e. attraction to young), and because the good parent process, which favors traits that signal parental care ability, would constrain the evolution of costly secondary sexual characteristics due to evolutionary tradeoffs between parental investment and sexually selected traits. Using a phylogenetic comparative approach, we studied plumage and bare-part characteristics of adults in relation to brood parasitism in cuckoos (family Cuculidae), in which brood parasitism together with loss of parental care has evolved three times. As predicted, we found that non-parasitic cuckoos had plumage more similar to the juveniles than did brood parasitic cuckoos. Furthermore, non-parasitic cuckoos had a higher probability of having additional bare skin, i.e. a seemingly less-costly, hatchling-like trait, than did brood parasitic cuckoos. This finding further supports the link between parental care and sexual selection, although the influence of a parasite-host interaction cannot be excluded. The analysis of evolutionary pathways suggested inter-dependent evolution of additional bare skin and brood parasitism. Brood parasitism together with the loss of parental care may facilitate the maintenance of a modest phenotype similar to the young, and vice versa in some cases.