Synchronization of Breeding and Intraspecific Interference in the Carrion Crow

Abstract

DARLING (1938) suggested that synchrony of laying in colonial birds is stimulated by between birds, and larger colony, greater stimulus. Hence laying in larger colonies will be more synchronized than that in small ones. In Darling's words, the degree of breeding synchrony by stimulation may be densityrelated (Darling 1938: 53). Since then various authors have examined these ideas for different species of birds. Some students found such density correlated onset of laying to occur (Coulson and White 1960 in Black-legged Kittiwake, Rissa tridactyla; Horn 1970 in Brewer's Blackbird, Euphagus cyanocephalus; Nelson 1967 in Gannet, Morus bassana), while others found that they could not confirm phenomenon in various species of gulls, Larus [spp.] (MacRoberts and MacRoberts 1972 in L. argentatus and L. fuscus; Vermeer 1963 in L. glaucescens; Weidmann 1956 in L. ridibundus). In addition to these observations, Ashmole (1963), Hailman (1964), and Harris (1970) suggested that display interaction or social stimulation' cause breeding synchrony in some 'non-annual breeders (the Sooty Tern, Sterna fuscata, and Swallow-tailed Gull, Creagrus furcatus, respectively). Darling (1938) suggested that synchronous laying decreased predation, and Brown (1967), Parsons (1971), Patterson (1965), and Kruuk (1964) proved that Herring, Lesser Black-backed, and Black-headed Gulls (Larus argentatus, L. fuscus, and L. ridibundus respectively) that breed at peak of breeding season had better breeding success than individuals that breed earlier or later than peak. Patterson (1965) also showed that Black-headed Gulls that breed outside or on edge of a colony were more vulnerable to predation and thus less successful than birds in center. Edge effect (i.e. lower breeding success on edge of a colony) was reported for Gannet (Nelson 1967) and kittiwake (Coulson and White 1956). Horn (1968) found that clumped nesting in Brewer's Blackbird improves foraging efficiency and predation avoidance only when colony is built in a large expanse of nesting habitat, surrounded by abundant, but patchily distributed, food. All species mentioned breed in dense colonies, where distance between closest nest is rarely more than a few meters, but there is no reason why territorial species should not synchronize breeding in order to benefit in way that some colonial birds do. For example,