Black-Headed Gull or Black-Headed Girl? On the Advantage of Concealing Sex By Gulls and Other Colonial Birds

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Abstract
Mechanisms for sex-recognition in the Black-headed Gull, Larus ridibundus, were analysed in order to understand the occurrence of homo-sexual male pairs among cap- tive members of this species. No discontinuities between males and females could be detected in either external characteristics or behaviour. Although most individuals select a mate of the opposite sex, there is no evidence that individuals which do not belong to the same pair are able to determine each other's sex. The adaptiveness of concealing sex for colonial monogamous species is discussed. Its significance was tested in a group of captive gulls. Small female-like males appeared to be most suc- cessful in reproduction. The development of polygynous mating systems seems to be opposed by a preference of females for caring males.

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