Obligate avian brood parasitic species impose the costs of incubating foreign eggs and raising young upon their unrelated hosts. The most common host defence is the rejection of parasitic eggs from the nest. Both egg colours and spot patterns influence egg rejection decisions in many host species, yet no studies have explicitly examined the role of variation in spot coloration. We studied the American robin Turdus migratorius, a blue-green unspotted egg-laying host of the brown-headed cowbird Molothrus ater, a brood parasite that lays non-mimetic spotted eggs. We examined host responses to model eggs with variable spot coloration against a constant robin-mimetic ground colour to identify patterns of rejection associated with perceived contrast between spot and ground colours. By using avian visual modelling, we found that robins were more likely to reject eggs whose spots had greater chromatic (hue) but not achromatic (brightness) contrast. Therefore, egg rejection decision rules in the American robin may depend on the colour contrast between parasite eggshell spot and host ground coloration. Our study also suggests that egg recognition in relation to spot coloration, like ground colour recognition, is tuned to the natural variation of avian eggshell spot colours but not to unnatural spot colours.

Departamento de Ecología Genética y Evolución Facultad de Ciencias Exactas y Naturales Universidad de Buenos Aires C1428EHA Buenos Aires Argentina

Department of Biology Long Island University Post Brookville New York NY 11548 USA

Department of Psychology Hunter College and the Graduate Center of the City University of New York New York NY 10065 USA

Department of Psychology Hunter College and the Graduate Center of the City University of New York New York NY 10065 USA Department of Animal Biology School of Integrative Biology University of Illinois at Urbana Champaign Champaign IL 61801 USA

Department of Zoology and Laboratory of Ornithology Palacký University 77146 Olomouc Czech Republic

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