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A comparative study of the reproductive behaviour and natural history of three sympatric species of cormorants, Phalacrocorax auritus,P. penicillatus, & P. pelagicus. Van Tets, Gerrard Frederick

Abstract

During the four months of summer of 1957 and 1958 the reproductive behaviour and natural history of three sympatric species of cormorants, phalacrocorax auritus, P. pelaqicus, and P. penicillatus, was studied at Mandarte Island, B.C. The three species were found to select different sites for perching and nesting, and to use different habitats for feeding. A description is given of the external colouration of the various age groups of the three species, based in part on banded birds of known ages. It was found that P. auritus and and P. pelagicus are polymorphic prior to breeding and almost monomorphic after the breeding season. No such polymorphism was found in P. penicillatus. The voices of P. auritus, and P. pelagicus include several destinct calls, but P. penicillatus has only one call note. On the other hand only P. penicillatus can flash its gular pouch. The various comfort movements and postures were found to be identical to those of other birds. During locomotion specific differences due to anatomy and habitat adaption became apparant. These differences were further emphasized as species specific signals during social contact. The courtship sequence was found to be functionally identical for all cormorant species studied, but to differ specificly at each step in the sequence, where a signal is used. Except for differences due to size and nesting habitat, the method of incubating the eggs and raising the chicks was found to be identical in P. auritus, and P. pelagicus. It was concluded, that the species specificity of the signals, during courtship and social contact, serves to prevent hybridization, and thus preserves the divergent ecological specialization of the species. This in turn permits a more complete use of the environmental resources.

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