The nest-site preferences for seabirds of the northeast Pacific Ocean are given in Table 7, and Table 8 indicates the proportion of seabirds that belong to specific nest-site categories. 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 Washington 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 unstable. 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 have been gathered by aerial surveys. Consequently, the burrow and rock crevice categories are underestimated. The aerial survey is appropriate for only about 43% of the birds nesting on the Washington coast.
| 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's auklet | |
| 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 |
Northern and southern British Columbia provide another good example of habitat availability as revealed through seabird population 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 burrow. Indeed, 96% of the seabird population consists of nocturnal, burrow-nesting species. In southern British Columbia, however, there are more open-nest species, particularly glaucous-winged gulls and cormorants.
Overall, 68% of the breeding seabirds found along the northeastern Pacific coast are nocturnal and nest in burrows or rock crevices (Table 8). The most conspicuous nesting birds such as gulls, cormorants, and murres, comprise only 22% of the total population. Consequently, our current estimates of breeding seabirds still underestimate the more secretive, nocturnal, burrow-nesting species.
| Site | Estimated number of pairs | Percent of population | Total | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
| British Columbia | San Juan Islands | Washington coast | British Columbia | San Juan Islands | Washington coast | |||||
| Northern | Southern | Northern | Southern | Population | Percent | |||||
| Burrow-rock crevice | ||||||||||
| Diurnal | 1,849 | 11,334 | 231 | 7,504 | 2.0 | 18.1 | 1.4 | 17.3 | 20,918 | 9.7 |
| Nocturnal | 90,347 | 30,600 | 9,800 | 17,070 | 96.0 | 48.9 | 58.6 | 39.4 | 147,817 | 68.1 |
| Open nests | ||||||||||
| Flat or slope | 909 | 15,101 | 6,298 | 5,755 | 1.0 | 24.2 | 37.6 | 13.3 | 28,063 | 13.0 |
| Cliff face | 982 | 5,525 | 395 | 12,945 | 1.0 | 8.8 | 2.4 | 30.0 | 19,847 | 9.2 |
| Total | 94,087 | 62,560 | 16,724 | 43,274 | 216,645 | |||||
Population Changes
The available data are inadequate to detect changes in population distribution and density for most species (Table 9). In Washington, for instance, limited unsubstantiated information suggests an overall decline of the double-crested cormorant and tufted puffin in the San Juan Island area. Likewise, there seems to be an increase in glaucous-winged gulls there. In British Columbia, Drent and Guiguet (1961) were able to detect changes in some species. For example, they noted increases in the double-crested cormorant, pelagic cormorant, and glaucous-winged gull. No change was observed in the tufted puffin. Since then, the Brandt's cormorant has established a colony in Barkley Sound (Guiguet 1971). The data in southeastern Alaska are inadequate for all species except, perhaps, the Cassin's auklet which Gabrielson and Lincoln (1959) reported to be declining throughout Alaska. In short, no definitive statements can now be made concerning changes in seabird population numbers.