KOOPMAN K (1990) Sex ratio in Black-headed Gulls Larus ridibundus in the northern part of The Netherlands. LIMOSA 63 (3): 89-93.

This paper shows a simple but reliable graphical method to distinguish the both sexes of the Black-headed Gull Larus ridibundus . The head- + bill-length was plotted on probability-paper and birds with a head- + bill-length> 81.0 mm are males, the others birds females. Mist-net trappings carried out along the coast of the Dutch Wadden Sea and on inland roosts in April-October showed an equal sex ratio, trapping by a clap-net in October-March, however, yielded more males. Moreover, there are more ringing recoveries of male breeding birds than of female breeding birds outside the breeding season. It is suggested that the discrepancy between the numbers of trapped males and females is caused by the habitat and not by a difference in migration patterns of both sexes. Males are found more in urban habitats whereas females occur especially in more natural habitats. Probably both sexes use spatially separated foraging areas. The somewhat larger and more dominant males should displace the females from urban areas with an irregular and limited food supply to more natural habitats.

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