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Named after Adolphus Lewis Heermann, the 19th century explorer and naturalist, this medium-sized gull is unlike any other North American gull in both appearance and migration pattern. The white head of breeding adults transitions to slate gray at the neck and down to the belly, and the red bill is tipped with black. Genetic evidence (1) shows that Heermann's Gull belongs to the 'White-headed species' group of the Holarctic region. However, its exact position within this group is not well-resolved (2).
Gregarious by nature, Heermann's Gull forms breeding colonies on arid islands, mainly in the Gulf of California, Mexico, and a few have been recorded nesting on islands along the Pacific coasts of Mexico and California (United States). The nesting season extends from March through July, depending on the location of the colony. The largest colony is on Isla Rasa, where an estimated 90–95% of the world population breeds. After breeding, most adults and young migrate from the Gulf of California to the Pacific coast and spread south to southern Mexico and north to southern British Columbia, with peak numbers arriving in Canada during July and August. These northern individuals then retreat south to winter along the coast only from California south, with some nonbreeding immatures remaining along the coast year-round.
During the nonbreeding season, some individuals feed along beaches, sheltered bays and harbors, rocky promontories, and kelp beds, but the majority are maritime and pursue schools of herring many miles offshore. During the nesting season, in the Gulf of California, they feed mainly on small pelagic fish, most importantly Pacific sardine (Sardinops sagax), northern anchovy (Engraulis mordax), and Pacific mackerel (Scomber japonicus) (3, 4, 5). They often join mixed-species assemblages of feeding cormorants (Phalacrocorax spp.), boobies (Sula spp.), and Brown Pelican (Pelecanus occidentalis), and kleptoparasitize food, particularly from Brown Pelicans, by snatching fish directly from the pelican’s pouch. Nests consist of simple scrapes on the ground sometimes lined with debris. Clutch size ranges from 1–3 eggs, which varies according to food abundance and female age (6). After 28 days of incubation, semiprecocial chicks hatch and fledge at approximately 45 days of age.
Historically, Heermann's Gull was harvested by Mexican fisherman and Native American egg collectors; an estimated 50,000 eggs were removed during one breeding season at Isla Rasa (7). Other threats have included high levels of pesticide contaminants in body tissues accumulated through diet during the 1980s (8), direct competition with humans for Pacific sardine (3), human disturbance at breeding colonies (9), and introduction of nonnative vertebrates such as black rat (Rattus rattus) and house mouse (Mus musculus). However, more recent studies have detected lower levels of pesticides in body tissues (Anderson et al., unpublished data), and nonnative rodents were eradicated from Isla Rasa in 1995 (7). With establishment of Isla Rasa as a seabird sanctuary by the Mexican Government in 1964, protection has been afforded to the most important breeding colony. From an estimated low of 55,000 pairs in 1975, the global population is currently estimated at 350,000 individuals. This increase may account for recent nesting along the California coastline and an increase in extralimital records in North America in recent decades.
Long-term research at Isla Rasa, Gulf of California, Mexico, has provided the only detailed information on diet (10, 3, 4, 11, 5). Aspects of egg water loss, temperature regulation, and oxygen consumption in both adults and embryos on the arid islands have been documented (12, 13, 14). Other detailed studies on Isla Rasa include density estimates of breeding colonies (15), breeding biology (16, 17, 15, 18), behavioral postures associated with incubation and thermoregulation (19), interactions with other species (20, 21), population estimates at major breeding sites (22, 5), demographic history (23), life history strategies (24), and human impact at breeding colonies (19, 25, 18, 20, 15).