Breeding Biology of Heermann's Gulls on Isla Rasa, Gulf of California, Mexico

Abstract

During 1980 and 1981, I studied nest-site selection and reproductive success in Heermann's Gull (Larus heermanni) on Isla Rasa in the Gulf of California, Mexico. About 95% of the world population of this species nests on Isla Rasa. In 1980, about 130,240 nests were present within an area of 0.8 km 2. In the Midriff Island Region (which includes Isla Rasa), oceanic productivity is high even during E1 Nifio events because of tidal upwelling. My results indicate that the Heermann's Gull is characterized by high nesting density (up to 110 nests per 100 m 2) and high nesting synchrony. Massive synchronized communal flights to nesting areas occurred every evening during several weeks prior to egg laying. Nesting density in valleys (71 per 100 m ) was higher than on rocky hills (9.5 per 100 m2). Clutch size was higher in valleys in 1981 than in 1980 and was lowest on rocky substrates on hills. Hatch- ing success was independent of clutch size but varied with substrate and year. Fledging suc- cess among broods was independent of hatching order. Fledging success and reproductive success (i.e. the number of chicks fledged/number of eggs laid) were higher in valleys in 1981 and were lowest on rocky hills in 1980, and reproductive success was significantly high- er for three-egg clutches than for one-egg clutches. The observed patterns of reproductive performance may be correlated with annual variation in food abundance and with exposure to predators in different nesting habitats and among different nesting densities. Received 2 December 1996, accepted 30 October 1998.