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Status: Population size: 525,000 Photo by Jo Anne MacKenzie.

Heermann's Gull
(Larus heermanni)

One of North America's most beautiful gulls, Heermann's Gull is a unique bird of the Pacific Coast. Almost the entire global population of this species breeds on one island group off the coast of western Mexico - following breeding season, these gulls disperse northward along the coast as far as southern British Columbia. Surprisingly aggressive for birds their size, Heermann's Gulls steal fish from the pouches of Brown Pelicans and actively chase other birds to dislodge prey items. Because of their heavy reliance on a single island nesting site, Heermann's Gulls are especially susceptible to disturbances such as egg harvesters and introduced predators.

Identification
Among North American gull species, Heermann's Gulls are unique in their appearance: breeding adults have light gray underparts, dark gray upperparts, a white head, and a bright red bill with a black tip. This species is similar in size to the common Ring-billed Gull.

Distribution and Population Trends
Heermann's Gulls nest on islands along the coast of western Mexico from February to May, and then move northward to the Pacific Coast of the United States following their breeding season. Outside of the nesting season, the species can be found as far north as southern British Columbia, and it may also disperse southward from its Mexican breeding grounds. In Mexico, there are several Important Bird Areas that protect Heermann's Gull breeding grounds, including Isla Benitos, Isla Angel de la Guarda, and Isla Rasa, which supports 300,000 breeding birds; this number represents 95% of the world's breeding population.

Ecology
This species is strictly a coastal bird, found along beaches, rocky shoreline, estuaries, and lagoons. Birds nest in colonies on hot desert islands off the coast of northwestern Mexico. Nest construction by Heermann's Gulls ranges from simple scrapes in the soil to feather-lined cups made of grass. Nests typically contain two to three eggs, which are incubated by both sexes. Incubation period is not definitely known, but is probably about 28 days. Following hatching, the young birds are fed by both parents.

Despite being only a medium-sized gull, this species is quite aggressive, and can often be seen harassing other birds to make them drop food items. In addition, Heermann's Gulls will try to steal fish directly from the pouches of Brown Pelicans. In fact, the post-breeding dispersal of Heermann's Gull coincides with the northward movement of Brown Pelicans. These gulls also forage on their own, catching small fish near the surface of the sea.

Threats
The main threats to Heermann's Gulls come on their island nesting grounds, where they are vulnerable to disturbance. Specific threats include harvesting of eggs by fisherman, nest predation by introduced mammals, industrial development for guano extraction, and tourism.

Conservation
The island of Isla Rasa, which serves as the breeding ground for 95% of the world's population of Heermann's Gulls, is included in the Islas del Golfo de California Biosphere Reserve, as is Isla Angel de la Guarda, home to another breeding colony.

What Can You Do?
Volunteers are crucial to the success of programs that monitor the status of populations of Heermann's Gulls and other bird species. Audubon's Christmas Bird Count (CBC) is one of the longest-running citizen-science monitoring programs in the world and has helped to follow changes in the numbers and distribution of Heermann's Gulls. To learn more about the CBC and how you can participate, visit: http://www.audubon.org/bird/cbc.

Information on where Heermann's Gulls occur and in what numbers is vital to conserving the species. Help in monitoring this and other species by reporting your sightings to eBird. A project of Audubon and the Cornell Lab of Ornithology, eBird is the world's first comprehensive on-line bird monitoring program: http://www.audubon.org/bird/ebird/index.html.

CIPAMEX, Audubon's BirdLife International partner in Mexico, has an Important Bird Areas program that is working to protect breeding habitat for Heerman's Gulls and many other species. To learn more about Mexico's Important Bird Areas program and how you can help visit: http://132.248.79.80/wwwcampus/cipamex/

References
del Coro Arizmendi, Ma. and L. M. Valdelamar. 2000. Areas de Importancia para la Conservacion de las Aves en Mexico. Cipamex AC, Mexico City.

Kaufman, Kenn. 1996. Lives of North American Birds. Houghton Mifflin Company, New York.

Sibley, David A. 2000. The Sibley Guide to Birds. Alfred A. Knopf, New York.