Contrasting egg recognition between European and Asian populations of tree sparrows (Passer montanus)

• Tree sparrows (Passer montanus) in Europe can recognize and reject foreign eggs and even mimetic conspecific eggs.
• European and Asian tree sparrow populations differ in egg recognition capacity.
• Artificial model eggs, common cuckoo eggs (Cuculus canorus) and eggs of seven passerines showed absence of egg recognition in Asian tree sparrows.

Although many biological phenotypes are generally regarded as consistent across the distributional range of a species, some traits such as egg discrimination behavior have been shown to display extensive intraspecific variation as a response to selection from brood parasitism. We investigated the egg recognition ability in an Asian population of tree sparrows (Passer montanus), and we compared that with the ability to recognize and reject intraspecific foreign eggs in a population in Europe. Extensive artificial parasitism with model eggs and real eggs of eight sympatric birds that vary in background color and markings revealed that egg recognition capacity is completely absent in this Asian population of tree sparrows. This result contrasts with previous studies in European populations showing extensive ability for discriminating between own and foreign eggs. Different evolutionary equilibria or differences in the risk of conspecific parasitism may account for differences in egg discrimination ability between European and Asian populations of tree sparrows.