Are dark cuckoo eggs cryptic in host nests?

The coevolutionary arms race between cuckoos and their hosts has famously yielded cuckoo eggs that evade host recognition and rejection by mimicking the appearance of the host's own clutch. But not all coevolutionary interactions between cuckoos and hosts have followed the same pathway. Several host species do not show egg rejection even when the cuckoo's egg looks entirely unlike their own. For example, hosts of some Australian bronze-cuckoos, Chalcites spp., routinely accept olive-brown cuckoo eggs that look very different from the speckled white eggs of their own clutch. Here we investigate the hypothesis that these cuckoo eggs are cryptic, which might explain why hosts do not remove them from their clutch. First, we use a phylogenetic analysis to show that dark bronze-cuckoo eggs are not ancestral, but instead have evolved in a group that parasitizes hosts with dark nests exclusively. Second, we show that dark bronze-cuckoo eggs are laid by two species that parasitize hosts with relatively dark nests, whereas a congeneric bronze-cuckoo species parasitizing host nests with greater ambient light levels lays a mimetic egg. Finally, we use a model of avian visual processing to show that the dark eggs of Gould's bronze-cuckoo C. russatus are cryptic in dark host nests. Our results support the hypothesis that some bronze-cuckoo species and their hosts have pursued an alternative coevolutionary trajectory, which has resulted in the evolution of cryptic, rather than mimetic, cuckoo eggs.