Heermanns gull

Appearance

This species looks distinctly different from other gulls. Adults have a medium gray body, blackish-gray wings and tail with white edges, and a red bill with a black tip. The head is dusky gray in non-breeding plumage and white in breeding plumage. Immatures resemble non-breeding adults but are darker and browner, and the bill is flesh-colored or pink till the second winter. A few birds, no more than 1 in 200, have white primary coverts, which form a showy spot on the upper wing. This gull is unlikely to be confused with other species as it is the only white-headed, gray-bodied gull found on the west coast of North America.

Calls are described as deep and similar in pattern to other gulls but noticeably different in quality.

Distribution

Of the current population of about 150,000 pairs, 90% nest on the island of Isla Rasa off Baja California in the Gulf of California, with smaller colonies as far north as California and as far south as Nayarit. After breeding, birds commonly disperse to central California, and less commonly north as far as British Columbia and south as far as Guatemala.

Status

Isla Rasa was declared a sanctuary in 1964. Egg-collecting and disturbance during the breeding season are discouraged. With the breeding colony concentrated on one small island this species is vulnerable to a catastrophic weather event. The success of the colony in any one year is dependent on the availability of prey and this is related to the ocean temperature changes brought about by El Niño. These factors have caused the IUCN to rate this bird as "Near Threatened".

Behavior

This species nests colonially on the ground, like many gulls. The nests are often at densities as high as 110 nests per 100 m2 . It lays two or three eggs, grayish buff, to buff with gray and brown markings.

Heermann's gull sometimes steals prey from other seabirds, particularly brown pelicans, with which it often associates.