California Colonies 



Catalog Total 

 18,304 birds 

 = colony sites 

 = former sites 



Percent of California 

 Breeding Population. 



Breeding Chronology 



Northern Cal.fornia 



laying 



CM*- 1 <"' 37 > 



Incubation Period - *2 days 

 Nestling Period - 66 -70 diy> 



(24) 



hatching 



egg laying 



hatching 



(31) 



hatching 



Vo ana available lor Cnannel Islands populations 

 Chronology constructed from dates of eygs y 



tfd in Lot Coronadot Islands of 

 MMKO. 



' 1' 1 iajaa^La^Lafaa^am 



fledging 



Jan. | Feb. | Mar. | Apr. | May June | July [ Aug. | Sep. | Oct. I Nov. | Dec. 



additional small colonies may exist on almost any 

 island with suitable habitat. 



HISTORICAL STATUS AND VULNERABILITY 



Limited published data suggest that Leach's 

 Storm-Petrels have declined in number in 

 California since the turn of the century. Breeding 

 colonies were once found on Whaler Island (325 

 045), Blank Rock (325 024), Flatiron Rock (325 

 023), and Green Rock (325 020). They have 

 been extirpated from the first three islands and 

 only one dead petrel was found on Green Rock 

 during a survey in 1970 (Osborne, 1972). On 

 Whaler Island, a colony of 20,000 storm-petrels 

 was destroyed after construction of a breakwater 

 to the island in the late 1 930's and the breakwater's 

 subsequent strengthening in the early 1950's. 

 The island was quarried for rock and the intro- 

 duction of rats hastened the colony's demise 

 (Osborne 1972). Storm-petrels on Blank, Flatiron, 

 and Green Rocks may have been reduced in 

 number or eliminated by over-collecting and 

 accelerated soil erosion caused by early egg 

 collectors and by erosion caused by nesting 

 cormorants and murres. 



Harris (pers. comm.) believes that the recently 

 established breeding population of Double-crested 

 Cormorants on Little River Rock (325 035) 

 is preempting and destroying storm-petrel nesting 

 habitat. This Double-crested Cormorant colony 

 has increased substantially in recent years, from 

 two pairs in 1974 to about 50 pairs in 1980 



(Yocom and Harris 1975, this study). Nearby 

 Trinidad Bay Rocks (325 054) may be receiving 

 the petrels displaced from Little River Rock. For 

 example, Button Rock (325 054) had no nesting 

 storm-petrels in 1970 (Osborne 1972), but in 

 1980 approximately 1,000 Leach's Storm- 

 Petrels were actively breeding there. Nevertheless, 

 the loss of nesting habitat through soil erosion 

 and the displacement of birds by cormorants may 

 be two of the most significant threats to storm- 

 petrels in California. 



Predators such as River Otters (Lutra 

 canadensis) and Mink (Mustela vison) can also be 

 detrimental to storm-petrel colonies. Osborne 

 (1972) found about 90 dead Leach's Storm 

 Petrels on Prisoner Rock during surveys in 1969, 

 1970, and 1972 and attributed these losses to 

 mink. In 1980 we observed 45 Leach's Storm - 

 Petrel carcasses on Prisoner Rock and suspect a 

 river otter was the predator. Despite this loss, our 

 estimate of Leach's Storm-Petrels for this colony 

 remains similar to Osborne's. The population of 

 Leach's Storm-Petrels on the Farallon Islands 

 appears relatively stable in size (Ainley and Lewis 

 1974). 



Leach's Storm-Petrels are vulnerable to 

 contamination by oil. The period of greatest 

 susceptibility is from February to October when 

 they are most abundant off coastal California. 

 During the winter they are uncommon within 

 30 kilometers of shore (Ainley 1976) and are 

 unlikely to be affected by coastal oil spills. 



23 



