Breeding Dynamics of Heermann’s Gulls

Abstract

Islands are landmarks for sea birds, whether for orientation, as resting points during foraging and migration trips, or most importantly as nesting sites. This is due to the isolation that islands offer, rendering them free of many of the continental predators. If, additionally, islands are located in the midst of highly productive waters, they provide sea birds with abundant food, which is particularly valuable during the nesting season. This is the case in the northern Sea of Cortes. Not surprisingly, we find that the islands of this region are nesting sites for more than 90% of the world’s populations of Heermann’s Gulls (Larus heermanni) and Elegant Terns (Sterna elegans), and for about 90% of the global populations of the Least Storm-petrel (Oceanodroma microsoma), the Craven’s Murrelet (Synthliboramphus craveri), and the Yellow- footed Gull (Lams livens). The midriff island area of the Gulf of California also shelters approximately 70% of the world’s Black Storm-petrel (O. melania) and, at the subspecific level, provides breeding grounds for about 50% of the California Brown Pelicans (Pelecanus occidentalis californicus), 50% of the Blue-footed Boobies (Sula nebouxii nebouxii), and 40% of the Brown Boobies (S. leucogaster brewsteri). A combination of characteristics in one particular island, Rasa, has made it a natural breeding sanctuary for Heermann’s Gulls. Besides the two traits mentioned above (lack of land predators and high marine productivity), these characteristics include (1) its characteristic flat topography from which it derives its name (rasa means “flat” in Spanish), and (2) its sparse vegetation cover, resulting from the extensive coverage of the island with guano that hinders vegetation growth. The Heermann’s Gull is the only North American representative of the group of White-hooded Gulls (Anderson 1983; Moynihan 1959; Storer 1971). The only other member of this group of gulls and hence its closest relative is the Grey Gull (Lams modestus), which inhabits the Pacific coast of South America along Chile and Peru and breeds inland in the Atacama Desert some 50-100 km away from the coast (Howell et al. 1974; Howell 1978).