California Colonies 



Catalog Total 

 64,210 birds 

 = colony sites 



Breeding Chronology 



Percent of California 

 Breeding Population. 



Jm. [ Fb. | M. | Apr. | M.y | Jun. | July | Aug. | Sp. | Oct. | N 



: I NOV.TD.C. 



et al. 1979). Hunt et al. (1979) speculate that 

 they were affected first by human disturbance 

 and later by the accumulation of pesticide residues. 

 Thin eggshells, similar to those caused by DDE 

 concentrations in Brown Pelicans and Double- 

 crested Cormorants on the Channel Islands, were 

 seen in Brandt's Cormorant eggs on San Nicolas 

 Island (524 01 1) and Lion Rock (477 Oil) (Hunt 

 etal. 1979, Frame 1972). 



Brandt's Cormorants on the Farallon Islands 

 declined in number during a period of high 

 human disturbance in the mid-1 800's. Populations 

 began to increase in size after this period of 

 exploitation, and by 1972 there were 22,000 

 Brandt's Cormorants breeding on the islands 

 (Ainley and Lewis 1974). About 28,000 Brandt's 

 Cormorants now occupy the Farallon Islands 

 (Ainley pers. comm.). 



Our population estimates for colonies along 

 the remainder of the California coast are nearly 

 double population figures from 1969 and 1970 

 (Osborne and Reynolds 1971). The difference 

 may be due, at least in part, to more accurate 

 censusing rather than to real population changes. 

 It may also reflect an unusually high nesting 

 effort in 1979. 



Changes in ocean conditions from year to 

 year apparently influence breeding populations of 

 Brandt's Cormorants. Periods' of upwelling, 

 which bring cold, rich waters to the surface and 

 trigger plankton blooms, are of great importance 

 to seabirds. When warmer ocean waters prevail, 



the quantity of food drops. Breeding populations 

 of seabirds become smaller and reproductive 

 success declines. Favorable ocean conditions in 

 1979 caused a boom year for Brandt's Cormorants 

 on the Farallon Islands, where up to 28,000 birds 

 nested. Conditions in 1980 were less favorable, 

 and only about 16,000 birds nested there (Ainley 

 pers. comm.). 



Brandt's Cormorants are vulnerable to 

 disturbance during the breeding season. Adults 

 flush from their nests readily when approached 

 by boats, low flying aircraft, or humans on foot. 

 Once parents are away from the nests, Western 

 Gulls are able to prey upon eggs and chicks. 

 Repeated disturbance can cause permanent 

 colony desertion. 



Reported cormorant deaths from oil spills 

 are not frequent (Berkner pers. comm.), and 

 Brandt's Cormorants made up only one percent 

 of the recovered birds in the 1971 San Francisco 

 oil spill (Smail et al. 1972). The relative low 

 number of oiled cormorants found on beaches 

 may reflect a greater tendency to sink than in 

 other seabird species (Harris per. comm.). 

 Brandt's Cormorants are likely the most vulnerable 

 of all cormorants to localized oil spills because of 

 their large breeding concentrations. . 



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