California Colonies 



Catalog Total 



362 birds 

 = colony sites 



28% 



Percent of California 

 Breeding Population. 



Breeding Chronology 



North. CilHomli 



Clutch - 1 ( 38) 



Incubation Period - 31 33 day. 

 Nulling Pfiod - 36 46 dys 



Fin Ion lilmdl 



? ; 

 hatching 



(14, 35) 



ftodjing 

 duta praam 



No Dm 



Numbers in parentheses correspond to the 

 '. references from which we obtained the data. 



Charm* Itlandi 



NotBTMding 



| Junl July | Am- | Sep. | Oct. | Nov. I Dc. 



HISTORICAL STATUS AND VULNERABILITY 



Rhinoceros Auklets were first found breeding 

 at Castle Rock (325 006) in 1917 when Clay 

 (pers. comm. in Osborne 1972) claimed to have 

 found one nest. Clay, who periodically visited 

 many of the seabird islands in northern California, 

 makes no further mention of this species. Osborne 

 (1972), after his 1969 and 1 970 surveys, estimated 

 a population of between 100 and 150 Rhinoceros 

 Auklets on Castle Rock. He revisited the island in 

 1977 and felt the population had increased to 

 200 birds (Osborne pers. comm.). To avoid 

 disturbing the seabirds at Castle Rock we 

 conducted no new groundwork there during this 

 study. 



Rhinoceros Auklets were known to breed on 

 the Farallon Islands in the early 1800's but 

 disappeared in the 1860's, possibly as a result of 

 collecting by overzealous scientists. They were 

 not observed there again until 1971, and in 1972, 

 two or three pairs were present (Ainley and 

 Lewis 1974). Presently about 100 Rhinoceros 

 Auklets are suspected to breed on the Farallon 

 Islands and the population is thought to be 

 expanding (Ainley pers. comm.). 



Before 1979, Rhinoceros Auklets were 

 known to nest only as far south as the Farallon 

 Islands. Although our observations at Point 

 Arguello do not constitute a breeding record we 

 are confident that they are breeding there or 

 will do so in the near future. This would represent 

 a range extension of some 350 kilometers. 



Rhinoceros Auklets are very sensitive to 



disturbance during the nesting period. Adults will 

 readily desert their nests if disturbed during 

 incubation or brooding. Their burrows are often 

 near the surface of the ground and are easily 

 collapsed. 



Like all alcids, Rhinoceros Auklets are 

 extremely vulnerable to oil spills. During the 

 breeding season, they concentrate at two sites: 

 Castle Rock and the Farallon Islands. Oil slicks 

 or other pollution in the vicinity of these colonies 

 could affect a large portion of California's breeding 

 population. During winter, California waters 

 support large numbers of Rhinoceros Auklets, 

 many from colonies further north (Briggs 1980). 



The available data indicate an increasing 

 population of Rhinoceros Auklets in California. 

 Significant increases in numbers have been ob- 

 served at the two largest colonies in the state, Castle 

 Rock and the Farallon Islands, and Rhinoceros 

 Auklets have been found breeding or are suspected 

 to breed at six additional sites. These recent 

 increases and the discovery of new nesting sites in 

 Oregon (Scott et al. 1974, Varoujean and Pitman, 

 1979D and British Columbia (Hatler et al. 1978) 

 indicate that this population increase may be 

 widespread along the west coast of North America. 



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