Breeding site characteristics and breeding success in Thick-billed Murres

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Abstract
Breeding site characteristics of Thick-billed Murres, Uria lomvia, differed significantly between Cape Hay, Bylot Island, and Coburg Island, N.W.T., Canada. Logistic regression analysis showed that at both colonies site characteristics had significant effects on breeding success. At Coburg Island the slope and width of the ledge were the most important factors, whereas at Cape Hay the number of walls and neighbours, and to a lesser extent the width of the ledge, best explained the variation observed in breeding success. Egg and chick mortality factors differed at each colony. At Coburg Island most eggs were lost through accidentally rolling off ledges, and at this colony birds on level sites on broad ledges were most successful. At Cape Hay most mortality of eggs and chicks occurred as a result of rock- and ice-falls; sites offering some protection were the most productive. Comparisons are made with other studies, and the role of breeding site quality in population regulation is discussed.