History and Status of Colonies of Heermann's Gull in Mexico

Abstract

-Heermann's Gulls (Larus heermanni) have been reported to nest on 19 islands in Mexico, but during 1999-2000 they bred on only twelve. Of the known active colonies, one, Isla Rasa, harbors over 95% of the breeding numbers, with between 300,000-400,000 adults. Three islands each support between 2,000 and 20,000 adults, and the rest from 4 to 2,000 adults. The colonies of this species have exhibited major variations through time, but data are inadequate to judge whether human activities or changes in oceanographic conditions were the cause of these variations. These two, along with size and distribution of nesting colonies, should be monitored, if the species' population dynamics are to be understood. Received 21 August 2000, accepted 19 September 2000.