Egg patterns as identity signals in colonial seabirds: a comparison of four alcid species

The ability to recognize mates, kin, offspring and neighbors by their individually distinctive traits—individual recognition (IR)—is widespread in animals. Much work has investigated IR from the perspective of the recognizer, but less is known about the extent to which signals have evolved to facilitate IR. To explore this, one approach is to compare putative identity signals among species that differ in life history and extent of IR. In Common Murres (Uria aalge), a colonially breeding seabird, the eggs of individual females are remarkably variable in terms of color and pattern (maculation). Common Murres also appear to recognize their own eggs, leading to the hypothesis that variable egg phenotypes evolved to promote recognizability. However, we lack a quantitative assessment of the egg pattern information in Common Murres and their close relatives. Here, we analyzed images of eggs laid by four alcid species: Common Murres, Thick-billed Murres (Uria lomvia), Razorbills (Alca torda) and Dovekies (Alle alle). We extracted pattern measures believed to be relevant to bird vision and calculated Beecher's information statistic (H_s), which allowed us to compare the amount of identity information contained in each species’ egg patterns. Murres, which nest in dense colonies and can recognize their own eggs, have egg patterns with a relatively large amount of identity information compared to Razorbills and Dovekies. Egg recognition has not been demonstrated in Razorbills and Dovekies, whose colonies are less dense. Our results are consistent with the hypothesis that complex patterns of Murre eggs may have evolved to increase individual recognizability.