Egg colour matching in an African cuckoo, as revealed by ultraviolet-visible reflectance spectrophotometry

Despite major differences between human and avian colour vision, previous s tudies of cuckoo egg mimicry have used human colour vision (or standards ba sed thereon) to assess colour matching. Using ultraviolet-visible reflectan ce spectrophotometry (300-700 nm), we measured museum collections of eggs o f the red-chested cuckoo and its hosts. The first three principal component s explained more than 99% of the variance in spectra, and measures of cucko o-host egg similarity derived from these transformations were compared with measures of cuckoo-host egg similarity estimated by human observers unawar e of the hypotheses we were testing. Monte Carlo methods were used to simul ate laying of cuckoo eggs at random in nests. Results showed that host and cuckoo eggs were very highly matched for an ultraviolet versus greenness co mponent, which was not detected by humans. Furthermore, whereas cuckoo and host were dissimilar in achromatic brightness, humans did not detect this d ifference. Our study thus reveals aspects of cuckoo-host egg colour matchin g which have hitherto not been described. These results suggest subtleties and complexities in the evolution of host-cuckoo egg mimicry that were not previously suspected. Our results also have the potential to explain the lo ngstanding paradox that some host species accept cuckoo eggs that are non-m imetic to the human eye.