Taxonomic significance of host-egg mimicry by facultative brood parasites of the avian genus Coccyzus (Cuculidae)

Abstract

Black-billed (Coccyzus erythropthalmus) and Yellow-billed (C. americanus) cuckoos are facultative brood parasites that occasionally lay their eggs in the nests of 10 and 11 other bird species, respectively. This study demonstrates that both cuckoo species produce blue–green eggs that fully or nearly match the eggs of over 70% of their reported host species, a proportion significantly greater than if hosts were being selected at random from the potential host pool. These results suggest that the cuckoos may be selecting hosts on the basis of their egg colour, and support a hypothesis of egg mimicry. Since egg mimicry is unlikely to evolve in a facultative parasite, its existence in Coccyzus would imply a historically intense relationship between these birds and their hosts. This hypothesis is corroborated by recent phylogenetic analyses which suggest that the ancestral Coccyzus was an obligate parasite. Factors responsible for the loss of obligate parasitism in this genus may also have contributed to the general paucity of obligate parasitism in New World cuckoos. Competitive exclusion or resistance to invasion by parasitic cowbirds (Molothrus spp.) should be considered.