| Summary: | We studied the ground colors and maculations of 161 Common Murre (Uria aalge) eggs laid by 43 females in 3 small breeding groups on the cliffs of Skomer Island, Wales, in 2016–2018. Both the colors and maculations varied much more among than within females, providing quantitative evidence for the egg traits that might facilitate the parents' ability to identify their own eggs on the crowded breeding ledges where the density is typically ∼20 eggs m–2. Ground colors had a trimodal distribution of hue values (whitish to pale brown, pale blue, or vivid blue-green) and maculations ranged from none to complex squiggles and blotches. The eggs laid by each female in different years were similar to one another, and replacement eggs laid by females within years were also more similar to their first egg than to other eggs in the same breeding group. Egg appearance did not differ among the 3 breeding groups that we studied. Our findings thus support anecdotal observations that, within and between years, female Common Murres lay eggs that have similar ground colors and maculations. We do not, however, find evidence that there is much difference among the eggs laid in different parts of a colony. LAY SUMMARY For more than a century, naturalists have marveled at the diversity of colors and markings on the eggs of Common Murres (Uria aalge) breeding in dense coastal colonies, where they lay and incubate a single egg. Largely anecdotal observations have suggested that each female lays distinctive eggs, and that eggs laid on the same cliff ledge tend to be more similar than expected if pairs are distributed randomly. We measured the appearance of 161 eggs laid by 43 Common Murres over a 3-year period on Skomer Island, Wales. Each female consistently laid eggs within a narrow range of whitish to pale brown (11 females) or pale blue to vivid blue-green ground color (32 females) and similar markings both within- and between breeding seasons. Despite offspring tending to return to breed near their parents, eggs laid within a few ...
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