The Cover. Thick-billed murres (Uria Lomvia; Alcidae) are cliff-nesting, arctic seabirds that breed in colonies of up to 300,000 pairs. Parentage studies in murres are challenging because sexually monomorphic partners lay a single egg on rocky cliff ledges without constructing a nest, pairs nest at high densities, adoption and alloparenting are observed, and kin groups occur in colonies. (Coats Island, Nunavut, Canada; photo by Kerry Woo. See Ibarguchi et al., pp 209-216.)
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