California Colonies 



Catalog Total 

 50,930 birds 

 = colony sites 



63% 



Percent of California 

 Breeding Population. 



Breeding Chronology 



(28,29) 

 Clutch -1-3 



Neitlmg Period - 42 45 d.yi 



ftodjing 



Chinnd liUndi 



(35) 



duln pnMnt 



(1) 



adults pfMent 



(28,29) 



Jan. | Feb. | M. | Apr. | Miy | Jyix | July | Aug. I Sp. | Oct. | Nov. | Pec. 



Populations of Western Gulls in California 

 have probably also increased in size during the 

 last two decades, but documentation is difficult 

 to obtain, since historical data for much of the 

 California coast are lacking. Osbome and 

 Reynolds (1971) censused many of the large 

 Western Gull colonies in central and northern 

 California, but their data may not be comparable 

 with ours. Certainly differing survey techniques 

 account for some of the differences between the 

 two sets of data. We were able to make a compari- 

 son of Osborne and Reynolds' data and our data 

 for 26 sites and found more Western Gulls at 20 

 sites in 1979 and 1980 than in 1970, fewer at four 

 sites, and no change at two sites. The apparent 

 net' increase in numbers of breeding Western Gulls 

 at these 26 sites is about 1 ,600 birds. In addition, 

 we found many previously undiscovered sites. 

 This probably reflects in part, our more complete 

 coverage of the coast. These comparisons suggest 

 that the number of Western Gulls in California 

 may have increased during the last decade. 



Other evidence suggesting that numbers have 

 increased comes from the Farallon Islands and 

 Prince Island (501 004). The number of Western 

 Gulls on the Farallon Islands plummeted in the 

 mid-late 1800's primarily because of egg collecting 

 and disturbance from domestic animals (Ainley 

 and Lewis 1974). The population recovered in 

 size to about 23,000 birds in 1959 and remained 

 stable until 1972 (Ainley and Lewis 1974). Since 

 then the population has increased to 32,000 birds 

 and there appears to be a surplus of adult-plumaged 

 birds. On Prince Island, a population of about 

 500 birds in 1968 (Huber 1968) increased to 

 about 1,000 birds in 1976 (Hunt et al. 1979). On 

 Santa Barbara Island, however, there has been a 



marked decline in numbers of Western Gulls since 

 1972, which appears to be related to the abundance 

 of anchovies (Hunt and Butler 1980). 



The effects of an increasing population of 

 Western Gulls on other seabirds are difficult to 

 assess. Western Gulls are the most important 

 predators of storm-petrels and Cassin's Auklets on 

 the Farallon Islands (Manuwal 1974, Ainley et al. 

 1974), and further increases in Western Gull 

 numbers may increase the rate of predation. 

 Increases in Western Gull numbers elsewhere in 

 California could have a similar effect on small 

 alcids and storm-petrels. Western Gulls klepto- 

 parasitize cormorants, Rhinoceros Auklets, and 

 probably Tufted Puffins. The present rate of 

 incidence is unknown but may increase if gull 

 populations continue to expand. 



This species is probably one of the least 

 vulnerable to oil spills of all seabird species 

 nesting in California, since they are highly mobile 

 and frequently return to land to rest and roost. 

 Western Gulls, like other surface nesting seabirds, 

 are susceptible to disturbances while nesting. 

 Disturbance in a particularly dense colony may 

 result in intraspecific pirating of eggs and canni- 

 balism. Chicks frightened from their territories 

 may be killed by neighboring gulls or become lost 

 and starve. Nevertheless, the future of the 

 Western Gull in California looks secure. With the 

 present pattern of waste disposal in California, 

 Western Gulls will probably continue to increase 

 in number. Because of their relatively high 

 reproductive potential, an excess of non-breeding 

 adults, and their ability to feed on a wide variety 

 of prey, they would likely make a rapid recovery 

 from any decline. 



39 



