Heermann's gull  (Larus heermanni)

Heermann's gull is distinctive with a dark grey-brown back and wings, pale grey underside and rump, and white head, tips to the tail and wing feathers. The bill is bright red and sometimes tipped with black. The legs are black. Non-breeding adults have grey or brown streaks on the head and immature birds are dark brown all over, with a pale base to the red bill (2).

Heermann's gull breeds off the coast of Mexico and in the Gulf of California, particularly on Isla Rasa, where the population has a stronghold. Very occasionally it is seen breeding in San Francisco Bay (2). Non-breeders spend the breeding season at the wintering sites along the coasts of California and British Colombia (3).

This coastal gull prefers beaches, rocky shores, estuaries and lagoons for foraging and breeding, and will often nest on offshore islands. During the winter it moves out to sea or remains in remote coastal areas (2) (3).

Whilst not overpoweringly large, Heermann's gull frequently harasses other seabirds into relinquishing their catch, picking mainly on the brown pelican (Pelecanus occidentalis) and the California sea lion (Zalophus californianus). It takes small fish from the water surface and sometimes dives to chase fish. It also scavenges and feeds opportunistically on lizards, insects, invertebrates and herring gull eggs along beaches, estuaries and marshes (2) (3).

Heermann's gull prefers to forage alone or in small groups, but during the breeding season it forms large colonies on the ground. Up to 100 pairs construct nests out of twigs and stones between rocks, lining the nest with feathers or shells. The female lays one to three spotted eggs in late April, which are incubated by both parents for 28 days. They hatch between mid and late May and the hatchlings are fed by the male and the female until they fledge 45 days later. If food is scarce, parents may choose one chick to feed, starving the other or even pecking it to death. Chicks that survive until adulthood begin to breed at three or four years of age (2) (3).

Concentrated into just eight colonies within a small area, Heermann's gull is considered to be stable, but faces risks inherent in such a compact community. Ninety percent of the world's population is found on Isla Rasa in the Gulf of California so conservation of this island habitat is crucial to the survival of this species. It is estimated that there are 150,000 pairs of Heermann's gulls living today, and that their main threat is the increasing population of the yellow-footed gull (Larus livens) which is a predator of Heermann's gull eggs and chicks (2).

Very little specific conservation action is targeted at Heermann's gull because although its range is small and it is therefore vulnerable, the population is judged to be stable. It has proved extremely difficult to survey, producing wildly different population figures for each census. However, it is important to keep some idea of the population size in order to ensure that the yellow-footed gull is not impacting negatively on Heermann's gull (2). Ilsa Rasa became a sanctuary in 1964 (3).

For further information on gull species see Del Hoyo, J., Elliott, A. & Sargatal, J. eds. (2001) Handbook of the Birds of the World Volume 3: Hoatzin to Auks. Lynx Edicions, Barcelona.

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