Also known as the White-headed Gull, and named after Adolphus Lewis Heermann, nineteenth-century explorer and naturalist, this medium-sized bird is unlike any other North American gull in appearance and migration pattern. The adult's white head shades abruptly into slate gray on the underparts, and the red bill is tipped with black. It is the northernmost member of a group of southern gulls and has close affinities to the Gray Gull (Larus modestus), with which it shares similar morphology and breeding biology.
Gregarious by nature, Heermann's Gull forms large breeding colonies on arid islands in the Gulf of California, Mexico, from March through July. The largest colony exists on Isla Raza, where an estimated 90–95% of the total world population breeds. Nonbreeders make their first appearance off the southern California coast during the last week of May, and they are later joined by breeders. A remarkable “reverse,” or northward, movement of these gulls occurs across almost the entire length of the Pacific coastline from San Diego, California, to Vancouver Island, British Columbia, with peak numbers arriving in Canada during July and August. This northward movement is then followed by a southward exodus during fall and winter, with some representatives of this species remaining year-round.
Although some individuals feed along beaches, sheltered bays and harbors, rocky promontories, and kelp beds, the majority are maritime and pursue schools of herring some kilometers offshore. They often join mixed-species assemblages of feeding cormorants (Phalacrocorax spp.), boobies (Sula spp.), and pelicans (Pelecanus spp.), and kleptoparasitize food, particularly from Brown Pelicans (P. occidentalis), by snatching fish directly from their pouches. The nest consists of a simple scrape on the ground sometimes lined with debris. Clutch size ranges from 1 to 3 eggs, but typically 1 or 2 eggs are laid. The semiprecocial chicks hatch after 28 days of in-cubation and fledge at 45 days.
Historically, Heermann's Gulls were persecuted by Mexican fishermen and native American egg collectors; an estimated 50,000 eggs were removed during one breeding year alone from Isla Raza. Recent threats to populations include accumulation of high levels of pesticide contaminants in body tissues through their diet, direct competition with humans for Pacific sardines (Sardinops sagax caeruleus), human disturbance at breeding colonies, and introduction of nonnative vertebrates such as black rats (Rattus rattus). In 1964, the Mexican government established a seabird sanctuary on Isla Raza, thereby protecting the major breeding colony. From an estimated low of 55,000 pairs worldwide in 1975, populations are currently estimated at 150,000 pairs. This increase may account for several recent nesting attempts along the California coastline and a smattering of extralimital records for North America during the last 35 years.
Heermann's Gull has been little studied; much remains to be discovered. Velarde et al. ( Velarde et al. 1994 ) provide the only detailed study of diet, conducted over several years at Isla Raza. Aspects of egg water loss and temperature regulation and oxygen consumption in both adults and embryos on the arid breeding islands have been documented ( Bennett and Dawson 1979 , Rahn and Dawson 1979 , Ellis and Frey 1984 ). Other detailed studies on Isla Raza include density estimates of breeding colonies ( Velarde 1999 ), breeding biology ( Boswall and Barrett 1978 ; Velarde Velarde 1983 , Velarde 1999 ; Urrutia and Drummond 1990 ), behavioral postures associated with incubation and related to thermoregulation ( Bartholomew and Dawson 1979 ), population estimates at major breeding sites ( Velarde and Anderson 1994 ), and human impact at breeding colonies ( Bartholomew and Dawson 1979 ; Anderson and Keith 1980 ; Urrutia and Drummond 1990 ; Velarde Velarde 1992 , Velarde 1999 ).