California Colonies^ 



Catalog Total 

 131,170 birds 

 = colony sites 



80% 



Percent of California 

 Breeding Population. 



Fb. I Mar. | Ape. | Miy | Juna | July | Aug. | Sp. | Oa. I Nov. | OK. 



Osborne (1972) estimated that numbers of 

 Cassin's Auklets on Castle Rock increased from 

 100 seen in 1959 by Thoresen (1964) to 3,600 in 

 1970. Cassin's Auklets formerly bred on Flatiron 

 Rock (325 023) as late as 1934 (Clay unpubl. 

 field notes). Osborne (1972) believes that soil 

 erosion was the principal reason for their extir- 

 pation from this island. 



Western Gulls prey heavily on Cassin's 

 Auklets at Castle Rock and the Farallon Islands 

 (Thoresen 1964). Young are pulled by gulls from 

 shallow burrows and adults are killed at night 

 when they unfortunately land at the feet of 

 roosting gulls (Thoresen 1964). 



Cassin's Auklets are vulnerable to disturbance 

 from humans and to the depredations of intro- 

 duced predators such as rats. Cassin's Auklets 

 may desert their nests if disturbed during 

 incubation and their burrows can easily be caved 

 in by unwary visitors to their colonies. 

 Fortunately, all colonies of Cassin's Auklets in 

 California except Green Rock (325 020) are 

 protected. 



Cassin's Auklets feed from the ocean surface 

 in large social flocks where they are highly 

 vulnerable to oil contamination (Hunt et al. 

 1979). An oil spill near the Farallon Islands, 

 where 80 percent of the state's population breeds, 

 would be particularly damaging to this species. 



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