"Jack-of-all-trades" egg mimicry in the brood parasitic Horsfield's bronze-cuckoo?

Parasitism of multiple host species by a generalist poses the difficulty of overcoming a potentially diverse array of host defenses. In some generalist avian obligate brood parasites, selection for egg mimicry by hosts has given rise to host-specific races (gentes), each of which lays an egg that mimics that of its favored host. However, it is unknown how generalist parasites lacking races are able to circumvent egg rejection by hosts. The Horsfield's bronze-cuckoo, Chalcites basalis, is a generalist brood parasite that exploits a diversity of hosts yet is reported to lay monomorphic eggs. Using reflectance spectrometry and visual modeling, we tested for egg polymorphisms in Horsfield's bronze-cuckoo eggs laid in the nests of 17 host species. We found that the host species possess broadly similar egg phenotypes that differ subtly but significantly from one another in their color and luminance. However, the Horsfield's bronze-cuckoo does not mimic this diversity, thus ruling out the existence of host-specific egg color and luminance phenotypes that are visible to birds but hidden from humans. Instead, our analyses support the idea that the Horsfield's bronze-cuckoo egg is a jack-of-all-trades mimic, lying in an intermediate position in avian visual space between the eggs of its various hosts. We suggest that jack-of-all-trades mimicry may be favored among brood parasites that parasitize hosts with a narrow range of egg phenotypes and where individual female brood parasites exploit multiple host species.