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One of the most striking outcomes of coevolution between species is egg mimicry by brood parasitic birds, resulting from rejection behavior by discriminating host parents. Yet, how exactly does a host detect a parasitic egg? Brood parasitism and egg rejection behavior provide a model system for exploring the relative importance of different visual cues used in a behavioral task. Although hosts are discriminating, we do not know exactly what cues they use, and to answer this it is crucial to account for the receiver's visual perception. Color, luminance (“perceived lightness”) and pattern information have never been simultaneously quantified and experimentally tested through a bird's eye. The cuckoo finch Anomalospiza imberbis and its hosts show spectacular polymorphisms in egg appearance, providing a good opportunity for investigating visual discrimination owing to the large range of patterns and colors involved …