SEABIRDS OF ALASKA, BRITISH COLUMBIA, AND WASHINGTON 



83 



Even though this does not represent all the 

 colonies, it covers the most important islands 

 and those islands where there appears to be 

 potential for seabird breeding. 



The major colony sites with more than 250 

 breeding pairs are located at Protection and 

 Smith islands, Bird Rocks, Colville Island, 

 Hall Island, North and South Peapod rocks, 

 Puffin Island, North Sisters, Viti Rocks, and 

 Williamson Rocks (Fig. 6). Glaucous-winged 

 gulls are the predominant species on all these 

 islands except Protection and Smith islands, 

 where there are large colonies of rhinoceros 

 auklets. Rhinoceros auklets (65%) and glau- 

 cous-winged gulls (32%) make up 97% of the 

 total San Juan Islands seabird population. 



Nest-site Preferences 



Food supply and availability of nest sites 

 are two critically important factors influenc- 

 ing the distribution and abundance of sea- 

 birds. Whereas information on general diet 

 composition is known for most seabird 

 species, we know little about the availability 

 of favored seabird prey. The dynamics of sea- 

 bird food chains is reviewed elsewhere in these 

 proceedings. 



The nest-site preferences for seabirds of the 

 northeast Pacific Ocean are given in Table 7, 

 and Table 8 indicates the proportion of sea- 

 birds that belong to specific nest-site cate- 

 gories. These preferences, in conjunction with 

 knowledge of the physical characteristics of 

 seabird habitat, permit a partial explanation 

 of the present distribution and abundance of 

 seabirds. For example, if we compare the San 

 Juan Island habitats with those of the Wash- 

 ington coast, it is apparent that there are 

 more cliff-nesting species on the coast. This 

 reflects the physical characteristics of the two 

 habitats. There are few cliffs in the San Juan 

 Islands, and those that exist are very un- 

 stable. Colony sites in the San Juan Islands 

 are typically on low, flat islands. Glaucous- 

 winged gulls are the most abundant nesting 

 species there. Coastal islands, on the other 

 hand, are either covered by dense vegetation 

 or are large monolithic chunks of rock with 

 few available flat areas. Population estimates 

 for the Washington coast are heavily biased 

 toward surface nesters, since most of the data 



Table 7. Nest-site preference for seabirds 

 breeding from Cape Fairweather, Alaska, 

 to the Columbia River, Washington. 



Nest-site type 



Bird species 



Burrow-rock crevice 



Diurnal Pigeon guillemot 



Horned puffin 

 Tufted puffin 



Nocturnal Fork-tailed storm-petrel 



Leach's storm-petrel 

 Kittlitz's murrelet 

 Ancient murrelet 

 Cassin'sauklet 

 Rhinoceros auklet 

 Open nests 



Flat or slope Double-crested cormorant 



Brandt's cormorant 

 Glaucous- winged gull 

 Herring gull 

 Western gull 

 Black oystercatcher 



Cliff face Pelagic cormorant 



Common murre 

 Black-legged kittiwake 



Tree branch Marbled murrelet 



have been gathered by aerial surveys. Conse- 

 quently, the burrow and rock crevice cate- 

 gories are underestimated. The aerial survey 

 is appropriate for only about 43% of the birds 

 nesting on the Washington coast. 



Northern and southern British Columbia 

 provide another good example of habitat 

 availability as revealed through seabird popu- 

 lation estimates. The population data are 

 more comprehensive and have largely been 

 gathered by island visitations. The islands in 

 the northern portion are heavily vegetated 

 and many have well-developed soil into which 

 storm-petrels, auklets, and murrelets can bur- 

 row. Indeed, 96% of the seabird population 

 consists of nocturnal, burrow-nesting species. 

 In southern British Columbia, however, there 

 are more open-nest species, particularly glau- 

 cous-winged gulls and cormorants. 



Overall, 68% of the breeding seabirds found 

 along the northeastern Pacific coast are noc- 

 turnal and nest in burrows or rock crevices 

 (Table 8). The most conspicuous nesting birds 

 such as gulls, cormorants, and murres, com- 

 prise only 22% of the total population. Conse- 

 quently, our current estimates of breeding 



