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

Hosts of obligate avian brood parasites use visual cues to distinguishbetween their own eggs and those of the parasite. Despite majordifferences between human and bird vision, most previous studieson cuckoo egg mimicry estimated color matching based on humancolor vision. Undetected by humans, ultraviolet reflectance(UVR) may play a previously ignored role for rejection behaviorin avian brood parasite systems. We explored this possibilityby manipulating UVR of great spotted cuckoo Clamator glandariuseggs and assessing the response of magpie Pica pica hosts. Wecoated cuckoo eggs with an ultraviolet (UV) light blocker thatreduced UVR but left the human visible reflectance (400–700nm) unaltered. The first control treatment also coated the eggsbut did not alter their reflectance. A second control groupof cuckoo eggs was maintained uncoated to control for handlingeffects on magpie discrimination. We artificially parasitizeda third of a breeding magpie population with each type of experimentalegg and studied the rejection of cuckoo eggs. We failed to findsignificant differences between rejection rate of cuckoo eggswith and without reduced reflectance in the UV region. Our resultsindicate that artificial reduction of UVR of cuckoo eggs doesnot affect the probability of ejection by magpie hosts.