Western Gull (Larus occidentalls) 



World Breeding Range 



Western Gulls are the breeding gull of the 

 California coast. They breed as far south as Baja 

 California, Mexico, and as far north as Washington 

 and southern British Columbia. Western Gulls 

 in Washington and British Columbia are sym- 

 patric with Glaucous-winged Gulls (Larus glau- 

 cescens) with which they hybridize extensively 

 (Hoffman et al. 1978). 



Western Gulls nest in a wide variety of 

 habitats, but usually are found in the greatest 

 densities on the flatter portions of islands and 

 offshore rocks. Some nest on the mainland 

 and these nests are always confined to areas 

 inaccessible to mammalian predators such as steep 

 slopes and cliff faces. Western Gulls will even 

 nest on man-made structures. Their nests are 

 substantial and are usually constructed of 

 vegetation gathered near the colonies. Western 

 Gulls usually lay clutches of from one to three 

 eggs, although in the Channel Islands clutches of 

 from four to six eggs result from female-female 

 pairing. The eggs in these supernormal clutches 

 are usually infertile and do not hatch (Hunt and 

 Hunt 1977). 



Western Gulls feed on a variety of prey. 

 Important food items from around the Farallon 

 and Channel Islands include anchovies, rockfish 

 (Sebastes), Pacific Sauries (Cololabis saira), 

 midshipmen (Porichthys), cephalopods, euphausi- 

 ids, barnacles, and offal (Ainley and Sanger 1979, 

 Hunt et al. 1979). Like large gulls elsewhere in 

 the world, Western Gulls often feed on human 

 refuse at dumps scattered along the coast. 



CALIFORNIA COLONIES 



Although Western Gulls breed widely along 



the California coast, most of the breeding popu- 

 lation is concentrated at a few sites. The 32,000 

 gulls of the Farallon Islands (429 012) make up 

 more than 60 percent of the entire breeding 

 population in California. Other large colonies in 

 California are at Middle Anacapa Island (502 008) 

 with 5,000 Western Gulls, Santa Barbara Island 

 (524 008) with 2,300 gulls, San Nicolas Island 

 (524 Oil) with 1,800 gulls, Castle Rock (325 

 006) with 1,350 gulls, and Prince Island (501 

 004) with 960 gulls. Most of the Western Gull 

 colonies in California, however, are much smaller. 

 Sixteen of the remaining sites have between 100 

 and 500 birds, 22 sites have between 50 and 100 

 birds, and 115 sites have fewer than 50 birds. 



HISTORICAL STATUS AND VULNERABILITY 



Western Gulls are probably the least likely of 

 California seabirds to suffer population declines 

 as a result of marl's activities. The population 

 may in fact be growing as the result of an abundant 

 food supply at garbage dumps. Increases in 

 numbers of large gulls may not be desirable since 

 these birds can harm other seabirds and interfere 

 with aircraft at airports. 



Increases in the size of several populations of 

 large gulls have been attributed to the availability 

 of human food wastes and sewage (Drury 1979, 

 Kadlec and Drury 1968, Vermeer 1963). Both 

 Herring Gulls (Larus argentatus) and Great 

 Black-backed Gulls (Larus marinus) in eastern 

 North America have increased in number and 

 caused substantial damage to tern and Atlantic 

 Puffin (Fratercula arctica) colonies by usurping 

 optimal nesting habitat, stealing food (klepto- 

 parasitism), and eating eggs and chicks (Nettleship 

 1972,Nisbet 1973). 



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