Ultraviolet reflectance of great spotted cuckoo eggs and egg discrimination by magpies

Hosts of obligate avian brood parasites use visual cues to distinguish between their own eggs and those of the parasite. Despite major differences between human and bird vision, most previous studies on cuckoo egg mimicry estimated color matching based on human color vision. Undetected by humans, ultraviolet reflectance (UVR) may play a previously ignored role for rejection behavior in avian brood parasite systems. We explored this possibility by manipulating UVR of great spotted cuckoo Clamator glandarius eggs and assessing the response of magpie Pica pica hosts. We coated cuckoo eggs with an ultraviolet (UV) light blocker that reduced UVR but left the human visible reflectance (400–700 nm) unaltered. The first control treatment also coated the eggs but did not alter their reflectance. A second control group of cuckoo eggs was maintained uncoated to control for handling effects on magpie discrimination. We artificially parasitized a third of a breeding magpie population with each type of experimental egg and studied the rejection of cuckoo eggs. We failed to find significant differences between rejection rate of cuckoo eggs with and without reduced reflectance in the UV region. Our results indicate that artificial reduction of UVR of cuckoo eggs does not affect the probability of ejection by magpie hosts.