California Colonies 



Catalog Total 

 1,884 birds* 



= colony sites 



10% 



Percent of California 

 Breeding Population. 



* coastal population only 



Breeding Chronology 



Northern California 



C.U.CH.2.7 '' 



Incubation Period " 26 - 27 days 

 Nestl.ng Period -35-42 days 



<*) 





: . (1) 



Channel Islands 



(29) 



' . adults present 



Jan. | Feb. | Mar. | Apr. | May | June | July | Au. | Sep. \ Oct. | Nov. J Pec 



been documented at the Old Arcata Wharf (325 

 038) colony in northern California (Ayers 1975). 



On the Farallon Islands, Double-crested 

 Cormorants were once the second most abundant 

 species of cormorant, numbering in the low 

 thousands (Ainley and Lewis, 1974). This 

 population of cormorants declined in size between 

 the 1800's and the early 1900's as a result of 

 disturbance by commercial egg collectors. The 

 population has not yet recovered, although egg 

 collecting stopped long ago. Its failure to do so 

 may be related to the disappearance of the Pacific 

 Sardine (Sardinops caerulea) from central and 

 northern California (Ainley and Lewjs 1974). 



We compared the number of Double-crested 

 Cormorants observed in the region north of Cape 

 Mendocino during our study with Osborne's 

 (1972) results and found that numbers had 

 increased at three sites. Nests were absent from 

 two other sites, but we found six additional 

 colonies unreported by Osborne. The total 

 number of Double-crested Cormorants in this 

 region has apparently increased from 530 in 1970 

 to 1,200 in 1980. These few data suggest that 

 Double-crested Cormorants may be increasing in 

 number on the north coast of California, but 

 since yearly variations in nesting effort occur and 

 cormorants have shown a tendency to switch 

 nesting islands often, caution in such speculation 

 is recommended. 



Human disturbance of Double-crested 



Cormorant colonies can be very destructive 

 (Ayers 1975). Cormorant eggs and chicks are 

 vulnerable to gull predation when adults are 

 frightened off their nests by human intrusion 

 (Kury and Gochfeld 1975). Inland colonies have 

 probably been disrupted to a greater extent than 

 any other Double-crested Cormorant colonies in 

 the state because of lake development and recrea- 

 tion (Kelly pers. comm.). 



Little is known of the vulnerability of 

 cormorants to oil, but few oiled birds have been 

 found after California oil spills (Smail et al. 1972, 

 Berkner pers. comm.). Cormorants are mobile 

 and it is likely they can avoid oil spills to some 

 degree. Unlike other seabirds, cormorants spend 

 large amounts of time out of the water. The 

 greatest numbers of Double-crested Cormorants 

 are found along the coast in winter, since many 

 inland breeding birds move to the coast at this 

 time of year. 



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