Abstract
FACTORS influencing the evolution of clutch size in birds have received notable attention from Kluijver1, Lack2, Wynne-Edwards3 and others. The family Sulidae (order Pelecaniformes) provides a good example of adaptive radiation within a group, different clutch sizes having evolved in a balanced ecological/behaviour system. This problem was studied on the Bass Rock, Scotland (1960–63), the Galapagos Islands (1964) and the Peruvian Guano Island of Guanape Norte (1964–5) for Sula bassana, S. sula, S. dactylatra, S. nebouxii and S. variegata (S. leucogaster data from Murphy4 and Dorward5). Data for S. abbotti are lacking, but this species probably closely resembles S. sula.
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