by
David A. Manuwal
College of Forest Resources
University of Washington
Seattle, Washington 98195
and
R. Wayne Campbell
Provincial Museum
Victoria, British Columbia, Canada
Current breeding seabird population estimates, nest-site preferences, and population changes are reviewed for southeastern Alaska, British Columbia, and Washington. There are 19 species of seabirds and a minimum of 216,566 pairs breeding in British Columbia and Washington. There are limited data on breeding populations for southeastern Alaska. Species diversity ranges from 17 species in Alaska to 15 species in British Columbia and 14 species in Washington. Eighty percent of all British Columbia seabirds breed on the east coast of Queen Charlotte Islands and the northwest coast of Vancouver Island. The three most numerous species in British Columbia are the fork-tailed storm-petrel, Oceanodroma furcata (31.3%); Cassin's auklet, Ptychoramphus aleuticus (24.6%); and ancient murrelet, Synthliboramphus antiquus (12.5%). In Washington, 74% (43,274 pairs) of the seabird population resides on the Olympic coast; the remaining 26% are in the San Juan Island area. About 54% of this population consists of the common murre (Uria aalge) and rhinoceros auklet (Cerorhinca monocerata). The rhinoceros auklet and glaucous-winged gull (Larus glaucescens) make up 97% of the total seabird population of the San Juan Islands. About 68% of all seabirds on the northeastern Pacific coast are nocturnal, burrow or rock crevice-nesting species. Currently available population data are inadequate to determine significant changes in population density for most species. Suggested topics for future research are presented.
The purpose of this paper is to discuss the known distribution, habitat, abundance, and status of breeding seabirds of the Alexander Archipelago in southeastern Alaska, the Province of British Columbia, Canada, and the State of Washington.
Even though several studies of the breeding biology of several seabird species in this area have been published, there have been few published surveys of known breeding colonies. In British Columbia the most extensive work has been done by the British Columbia Provincial Museum and the University of British Columbia (Drent and Guiguet 1961). Gabrielson and Lincoln (1959) summarized the available literature on Alaskan birds up to about 1958. Since then, no extensive surveys have been conducted in southeastern Alaska. The U.S. Department of the Interior (1972), in its environmental impact statement for the Trans-Alaska Pipeline, presented additional information on the seabirds of other parts of Alaska. In Washington, there are no published comprehensive surveys except those of Kenyon and Scheffer (1961) and unpublished surveys by the U.S. Fish and Wildlife Service and the University of Washington.
| Family | Common name | Regions | |||
|---|---|---|---|---|---|
| British Columbia | Washington | Southeastern Alaska | Total forms | ||
| Hydrobatidae | Storm-petrels | 2 | 2 | 2 | 2 |
| Phalacrocoracidae | Cormorants | 3 | 3 | 1 | 3 |
| Haematopodidae | Oystercatchers | 1 | 1 | 1 | 1 |
| Laridae | Gulls and terns | 2 | 2 | 3 | 4 |
| Alcidae | Auks, murres, puffins | 6 | 7 | 9 | 9 |
| Total | 14 | 15 | 16 | 19 | |
There are 19 species of seabirds that breed along the Pacific coast of southeastern Alaska, British Columbia, and Washington (Table 1). Southeastern Alaska has the largest number (17) of species. Errors in species identification are most likely with the Larus gulls, particularly in southeastern Alaska where the herring gull (L. argentatus) and glaucous-winged gull (L. glaucescens) breed in mixed colonies (Patten and Weisbrod 1974). A similar situation exists in Washington where the western (L. occidentalis) and glaucous-winged gulls intergrade (Scott 1971). Brandt's (Phalacrocorax penicillatus) and double-crested cormorants (P. auritus) are often difficult to identify from the air. This would be a problem in Washington and the southwest coast of Vancouver Island, where the two species are locally sympatric.
The area under consideration is the 400-km-long Alexander Archipelago (Fig. 1). This complex pattern of islands, bays, and inlets is characterized by extremely high precipitation and typical cool marine temperatures. Average annual precipitation in the Sitka area is 245.4 cm (1931-60), and the average annual temperature is 6.3°C (U.S. Weather Bureau 1974). As a consequence of this cool, humid environment, most of the islands are densely covered with conifers, chiefly Sitka spruce (Picea sitkensis) and hemlock (Tsuga heterophylla), and an almost impenetrable shrub cover composed of salmonberry (Rubus spectabilis), elderberry (Sambucus callicaipa), devil's club (Echinopanax horridus), and three species of Vaccinium (Heath 1915).
Fig. 1. Map of southeastern Alaska showing major seabird breeding colonies: 1—North Marble Island; 2—Forrester Island.
There are 16 species of marine birds breeding in the Alexander Archipelago. The major seabird breeding colonies are located at Glacier Bay and at St. Lazaria, Hazy, and Forrester islands (Fig. 1; Table 2). Published surveys of these colonies are available only for St. Lazaria (Willett 1912) and Forrester islands (Heath 1915; Willett 1915). Several authors have reported on seabirds from surrounding areas (Grinnell 1897, 1898, 1909; Swarth 1911, 1922, 1936; Patten 1974). There have been no surveys of seabirds of southeastern Alaska since before the 1940's (J. G. King, Jr., personal communication). However, since census data are available for only two colonies, we discuss them in more detail.
| Bird species | St. Lazaria Island | Forrester Island | ||
|---|---|---|---|---|
| Number of pairs | Percent of total | Number of pairs | Percent of total | |
| Fork-tailed storm-petrel | 2,000 | 8.0 | 10,000 | 6.0 |
| Leach's storm-petrel | 20,000 | 80.0 | 50,000 | 30.0 |
| Pelagic cormorant | 150 | 0.6 | 150 | 0.0 |
| Black oystercatcher | 4 | 0.0 | 50 | 0.0 |
| Glaucous-winged gull | 300 | 1.2 | 8,000 | 4.8 |
| Herring gull | 220 | 0.0 | ||
| Common murre | 300 | 1.2 | 20,000 | 12.0 |
| Pigeon guillemot | 150 | 0.6 | 300 | 0.0 |
| Ancient murrelet | 20,000 | 12.0 | ||
| Cassin's auklet | 2,000 | 1.2 | ||
| Rhinoceros auklet | 75 | 0.0 | 20,000 | 12.0 |
| Horned puffin | 12 | 0.0 | 1,100 | 0.7 |
| Tufted puffin | 2,000 | 8.0 | 35,000 | 21.0 |
| Total | 24,991 | 166,820 | ||
The studies by Willett (1912, 1915) and Heath (1915) provide some base-line information on species composition and abundance with which future studies on St. Lazaria and Forrester islands can be compared (Table 2). The somewhat greater species diversity on Forrester Island is primarily due to its greater size and more suitable soil type for ancient murrelets (Synthliboramphus antiquus) and Cassin's auklets (Ptychoramphus aleutica), species that are absent on St. Lazaria. Storm-petrels (Oceanodroma spp.) are the most numerous species on both islands, but there are proportionately more storm-petrels (88%) on St. Lazaria than on Forrester (36%). On the other hand, there are many large, burrowing alcids on Forrester Island. Nearly a third of the birds on Forrester are rhinoceros auklets (Cerorhinca monocerata), tufted puffins (Lunda cirrhata), and horned puffins (Fratercula corniculata).
The species composition of seabirds breeding on other islands is similar to that found on Forrester and St. Lazaria islands but less abundant. In Glacier Bay, for example, the only population data available are those provided by Patten (1974) for North Marble Island: pelagic cormorants, Phalacrocorax pelagicus (30 pairs); black oystercatchers, Haematopus bachmani (8); herring gulls (7); glaucous-winged gulls (500); common murres, Uria aalge (18); pigeon guillemots, Cepphus columba (60); horned puffins (4); and tufted puffins (30).
At the present time, it is impossible to draw any conclusions about changes in population density and distribution for most of the seabirds breeding in southeastern Alaska. Adequate data are available only for St. Lazaria and Forrester islands where Willet and Heath provided the only early extensive census data for this part of Alaska.
The rugged British Columbia coastline is characterized by 930 km of islands and inlets (Figs. 2, 3). With the exception of the inner southern portion, this coast is mostly uninhabited. The physical characteristics of the offshore islands are similar to those found off the Washington coast. Descriptions of some of these islands and the 15 species of breeding seabirds on them have been given by Drent and Guiguet (1961), Guiguet (1971), and Summers (1974).
A detailed analysis of British Columbia seabirds is not presented here since a more thorough analysis is in preparation by R. W. Campbell and R. H. Drent (manuscript). Instead, we present seabird population estimates available for the Province up to the summer of 1975; Tables 3 and 4 summarize these estimates for the five major portions of coastal British Columbia. The coast of British Columbia contains a myriad of small islands where there may be small numbers of breeding seabirds. Many of these have not been censused and are too numerous to include in Tables 3 and 4.
Fig. 2. Map of northern British Columbia showing sites of major seabird breeding colonies: 1—Skedans Island; 2—Limestone Island; 3—Agglomerate Island; 4—Bischoff Island; 5—Ramsey Island; 6—Alder Island; 7—Rankins Island.
Fig. 3. Map of southern British Columbia showing sites of major seabird breeding colonies: 1—Triangle Island; 2—Cleland Island.
More than half of the breeding seabirds in British Columbia are found on the east coast of the Queen Charlotte Islands, and the fork-tailed storm-petrel (Oceanodroma furcata) comprises more than half of that total. However, new unpublished data (K. Vermeer) for Triangle Island and the northwest coast of Vancouver Island indicate that the population figures in Table 3 for this area are underestimates. Nevertheless, these two regions have nearly 80% of all the breeding seabirds in the Province. This results from the very large populations of the rhinoceros auklet and tufted puffin on Triangle Island and the fork-tailed storm-petrel, ancient murrelet, and Cassin's auklet on various islands on the east coast of the Queen Charlotte Islands (Table 3).
Continuing surveys of breeding seabirds are being conducted by personnel of the British Columbia Provincial Museum and the Canadian Wildlife Service.
For this report, we have distinguished two major geographical areas in Washington where breeding seabirds are found—the western coast of the Olympic Peninsula and the San Juan Islands, including the Strait of Juan de Fuca.
On the Olympic Peninsula, seabirds breed on the offshore rocks, islands, and precipitous cliffs from Copalis Beach to Cape Flattery (Fig. 4). The offshore rocks and islands throughout this area (except Tatoosh Island) are now included in the Washington Islands National Wildlife Refuge. Most of the larger rocks and islands have dense stands of salmonberry, salal, and grasses, and a few support stands of stunted conifers (Fig. 5); most are inaccessible to man. The adjacent coast is dominated by the Olympic rain forest where the mean annual precipitation is about 337.1 cm (U.S. Weather Bureau 1956, 1965a, 1965b).
Because the San Juan Islands lie northeast of the Olympic Peninsula and east of Vancouver Island (Fig. 6) they are in a rain shadow; however, because of highly variable topography and aspect, most islands have a diverse assemblage of plant communities (Franklin and Dyrness 1973). Exposed south-facing slopes are occupied by grassland vegetation and frequently by scattered trees, usually Pseudotsuga menziesii and Arbutus menziesii. Most of the seabird colonies are located on rather small exposed islands with short, grassy, shrubby vegetation. In general, these islands are not suitable for burrowing species.
| Bird species | Straits of Georgia and Juan de Fuca | Southwest coast of Vancouver Island | Northwest coast of Vancouver Island | West coast of Queen Charlotte Island | East coast of Queen Charlotte Island | Prince Rupert to Queen Charlotte Island | Total birds | Percent of total |
|---|---|---|---|---|---|---|---|---|
| Fork-tailed storm-petrel | + | + | 1,050 | 97,100 | + | 98,160 | 31.3 | |
| Leach's storm-petrel | 10,000 | + | 1,800 | 180 | 750 | 12,730 | 4.1 | |
| Double-crested cormorant | 1,058 | 0 | 0 | 0 | 0 | 0 | 2,116 | >1.0 |
| Brandt's cormorant | 370 | 0 | 0 | 0 | 0 | 370 | >1.0 | |
| Pelagic cormorant | 2,174 | 336 | 3,350 | 1,456 | 496 | 12 | 9,998 | 3.2 |
| Glaucous-winged gull | 10,123 | 6,870 | 600 | 412 | 866 | 540 | 29,534 | 9.4 |
| Common murre | 16 | 3,000 | 0 | 0 | 0 | 3,016 | 1.0 | |
| Pigeon guillemot | 1,029 | 204 | 250 | 358 | 1,458 | 1,650 | 5,978 | 1.9 |
| Ancient murrelet | 0 | 0 | 200 | 42,150 | 4 | 42,354 | 13.5 | |
| Cassin's auklet | + | 50,000 | + | 26,500 | 450 | 76,950 | 4.6 | |
| Rhinoceros auklet | 1,200 | + | 10,000 | 300 | 200 | 11,700 | 3.7 | |
| Tufted puffin | 1 | 154 | 20,000 | 190 | 0 | 42 | 20,388 | 6.5 |
| Total | 14,385 | 19,150 | 77,200 | 15,466 | 169,050 | 3,648 | 313,294 | 81.2 |
| Geographic location | Population estimate | Percent of total |
|---|---|---|
| Straits of Georgia and Juan de Fuca | 14,385 | 9.2 |
| Southwest Coast of Vancouver Island | 9,575 | 6.1 |
| Northwest Coast of Vancouver Island | 38,600 | 24.6 |
| West Coast of Queen Charlotte Island | 7,733 | 4.9 |
| East Coast of Queen Charlotte Island | 84,530 | 54.0 |
| Prince Rupert to Queen Charlotte Strait | 1,824 | 1.2 |
| Total | 156,647 | 100.0 |
Fig. 4. Map of the Olympic Peninsula of Washington State showing sites of major seabird breeding colonies: 1—Protection Island; 2—Carroll Island; 3—Destruction Island.
In the Strait of Juan de Fuca, the two most important sites are Smith and Protection islands. Both are composed of glacial deposits and heavy sod that has developed under dense grassy vegetation (Fig. 7). Consequently, these two islands support most of the burrowing seabirds in the region. Unfortunately, both islands have historically been subjected to much human disturbance (Richardson 1961; Manuwal 1974).
The existing information on seabird colonies in both the coastal and San Juan Island areas has been largely derived from aerial surveys by the U.S. Fish and Wildlife Service. These surveys are inherently biased toward surface-nesting species such as gulls and cormorants. Population estimates for guillemots, auklets, storm-petrels, and puffins are less accurate. Some additional information obtained by direct island visitation has been provided by Kenyon and Scheffer (1961), Richardson (1961), Thoresen and Galusha (1971), G. Eddy (unpublished data), and D. A. Manuwal (unpublished data). Although other accounts of Washington seabirds are available, the references listed above are specifically oriented toward population assessment.
Despite the large number of offshore rocks, islets, and islands along the Pacific coast of Washington, significant seabird colonies are present only on about 30 islands. Since Table 5 summarizes the population estimates for 12 species of seabirds breeding on 24 major sites, it represents only the majority and not the total number of breeding seabirds on the Pacific coast of Washington. About 74% of the entire Washington seabird population resides on the coastal rocks and islands.
Major colony sites with more than 2,500 breeding pairs are Grenville Arch, Willoughby Rock, Destruction Island, Cake Rock, Carroll Island, and Bodelteh Island. More intensive censusing, especially of nocturnal burrowing species will undoubtedly raise the population estimates for these and other islands off the coast. About 54% of the total coastal population is composed of the common murre and rhinoceros auklet.
Fig. 5. Photograph of Destruction Island off the coast of Washington.
Fig. 6. Map of the San Juan Archipelago showing sites of major seabird breeding colonies: 1—Viti Rocks; 2—Colville Island; 3—Smith Island.
There are about 86 actual or potential seabird colony sites in this area; 25 (30%) are now considered important. Eleven islands are under Federal protection as National Wildlife Refuges. Part of Protection Island is owned by the Washington State Game Department to protect the largest rhinoceros auklet colony in the State. Most colony sites are on small islands with poorly developed soil which prevents burrowing species from using them. Consequently, the dominant species are surface nesters (such as gulls and cormorants) and rock-crevice nesters (like the pigeon guillemot). In all, about 31,000 seabirds of 7 species breed in the San Juan Island area. Breeding seabird population estimates for 49 of the 86 nesting sites are given in Table 6. 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.
Fig. 7. Photograph of Smith Island in the Strait of Juan de Fuca, Washington. The glacial deposits are evident from the composition of the cliff faces.
| Breeding site | Species | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Storm-petrels | Cormorants | Black oyster-catcher | Gulls | Common murre | Pigeon guillemot | Auklets | Tufted puffin | Total | |||||||
| Fork-tailed | Leach's | Unidentified | Double-crested | Brandt's | Pelagic | Western | Glaucous-winged western | Cassin's | Rhinoceros | ||||||
| Copalis Rock | — | — | 15 | — | — | — | — | 30 | — | — | — | — | — | — | 45 |
| Point Grenville | — | — | — | 60 | 30 | 80 | — | 165 | 40 | 1,100 | — | — | — | — | 1,475 |
| Grenville Arch | — | — | — | 30 | 20 | — | 1 | 60 | — | 3,000 | 4 | — | — | 3 | 3,118 |
| Flat Rock | — | — | 30 | — | — | — | — | — | 60 | 300 | — | — | — | — | 390 |
| Split Rock | — | — | — | 100 | — | — | 1 | 150 | — | 2,100 | 4 | — | — | — | 2,355 |
| Willoughby Rock | — | — | — | 80 | 40 | 15 | — | 150 | — | 3,000 | — | — | — | 25 | 3,310 |
| South Rock | — | — | — | — | — | 40 | — | — | 50 | — | — | — | — | — | 90 |
| Abbey Islet | — | — | — | — | — | 30 | 3 | — | 50 | — | — | — | — | 10 | 93 |
| Destruction Island | — | — | — | — | — | — | 12 | 350 | — | — | 25 | — | 10,940 | 350 | 11,677 |
| Middle Rock | — | — | — | — | — | 25 | — | — | 25 | — | 50 | — | — | — | 100 |
| North Rock | — | — | — | — | — | — | — | — | 25 | — | — | — | — | — | 25 |
| Alexander Island | — | — | — | — | — | 50 | 5 | — | 225 | — | — | — | — | 1,550 | 1,830 |
| Rounded Island | — | — | — | — | — | 25 | — | 25 | — | — | 1 | — | — | — | 51 |
| Giant's Graveyard | — | — | — | — | — | 10 | — | — | — | 150 | — | 50 | 150 | — | 360 |
| Quillayute Needles | — | — | — | 50 | 50 | 50 | — | — | 150 | 900 | — | — | — | 350 | 1,550 |
| James Island | — | 30 | — | — | — | 40 | — | — | 150 | 750 | 40 | — | — | 20 | 1,030 |
| Cake Rock | — | 500 | — | — | — | 150 | — | — | 600 | 300 | 12 | — | 50 | 1,000 | 2,612 |
| Sealion Rock | — | — | — | 70 | — | 30 | — | — | 250 | — | — | — | — | 5 | 355 |
| Carroll Island | — | 3,100 | — | — | — | 100 | 3 | — | 550 | — | — | 25 | 250 | 2,400 | 6,428 |
| Ball Rock | — | — | — | — | — | 50 | 7 | — | 150 | — | — | — | — | 750 | 957 |
| White Rock | — | — | — | — | — | 100 | — | — | 75 | 250 | — | — | — | 100 | 525 |
| Ozette Island | — | — | — | — | — | — | 1 | — | 15 | — | — | — | — | — | 16 |
| Bodelteh Island | 1,900 | — | — | — | — | 100 | 2 | — | 300 | — | 5 | — | — | 750 | 3,057 |
| Tatoosh Island | — | 25 | — | — | — | 100 | — | — | 1,500+ | 100 | 20 | 25? | 25? | 30 | 1,825 |
| Total | 1,900 | 3,655 | 45 | 390 | 140 | 995 | 35 | 930 | 4,215 | 11,950 | 161 | 100 | 11,415 | 7,343 | 43,274 |
| Breeding site | Species | |||||||
|---|---|---|---|---|---|---|---|---|
| Cormorants | Black oyster-catcher | Glaucous-winged gull | Pigeon guillemot | Tufted puffin | Rhinoceros Auklet | Total | ||
| Double-crested | Pelagic | |||||||
| Bare Island | — | 50 | 1 | 120 | + | — | 2 | 173 |
| Barren Island | — | — | — | — | — | — | — | 0 |
| Battleship Island | — | — | — | — | — | — | — | 0 |
| Bird Rocks | 30 | — | + | 320 | — | — | — | 350 |
| Cactus Island | — | — | 1 | — | — | — | — | 1 |
| Castle Island | — | — | — | — | 30 | — | — | 30 |
| Colville Island | — | 40 | 1 | 1,000 | — | — | — | 1,041 |
| Danger Island | — | — | — | 125 | 7 | — | — | 132 |
| Decatur Island | — | — | — | — | — | — | — | 0 |
| Eliza Island | — | — | — | 3 | 1 | — | — | 4 |
| Eliza Rock | — | — | — | 1 | — | — | — | 1 |
| Flat Top Island | — | — | — | — | + | — | — | + |
| Flower Island | — | 17 | — | 90 | — | — | — | 107 |
| Goose Island | — | — | — | 60 | — | — | — | 60 |
| Gull Rock | — | — | + | 125 | 7 | — | — | 132 |
| Gull Reef | — | — | — | — | — | — | — | 0 |
| Hall Island | — | — | 1 | 275 | — | — | — | 276 |
| Harbor Rock | — | — | — | — | — | — | — | 0 |
| Iceberg Island | — | — | — | — | — | — | — | 0 |
| Johns Island | — | — | 1 | — | — | — | — | 1 |
| Long Island | — | — | 8 | 80 | — | — | — | 88 |
| Low Island | — | — | 1 | 75 | 17 | — | — | 93 |
| Lummi Rocks | — | — | — | 4 | — | — | — | 4 |
| Matia Island | — | — | — | — | + | — | — | + |
| Mummy Rocks | — | — | — | 55 | — | — | — | 55 |
| Minor Island | — | — | — | 100 | — | — | — | 100 |
| O'Neal Island | — | — | — | — | — | — | — | 0 |
| Patos Island | — | — | — | 20 | + | — | — | 20 |
| North Peapod Island | — | — | 1 | 220 | 2 | — | — | 223 |
| South Peapod Island | — | — | 1 | 75 | 2 | — | — | 78 |
| Pearl Island | — | — | — | — | — | — | — | 0 |
| Pointer Island | — | — | — | 58 | 2 | — | — | 60 |
| Protection Island | 3 | 110 | 3 | 1,500 | 30 | 9,200 | 35 | 10,881 |
| Puffin Island | — | — | 1 | 350 | 15 | — | — | 366 |
| Ripple Island | — | — | — | — | — | — | — | 0 |
| Sentinel Island | — | — | — | — | 10 | — | — | 10 |
| Sentinel Rock | — | — | 1 | — | — | — | — | 1 |
| Skip Jack Island | — | — | — | 75 | 20 | — | — | 95 |
| Smith Island | — | 20 | 6 | 10 | 30 | 600 | — | 666 |
| Speiden Island | — | — | — | — | — | — | — | 0 |
| South Sister Island | 2 | 11 | 1 | 131 | — | — | — | 145 |
| Middle Sister Island | — | — | 1 | 22 | — | — | — | 23 |
| North Sister Island | — | — | 2 | 412 | 3 | — | — | 417 |
| Viti Rocks | 29 | 80 | 1 | 387 | 1 | — | — | 498 |
| Waldron Island | — | — | — | — | 2 | — | — | 2 |
| Williamson Rocks | — | 67 | 1 | 346 | 2 | — | — | 416 |
| Whale Island | — | — | 1 | 70 | — | — | — | 71 |
| White Rock | — | — | + | 125 | 13 | — | — | 138 |
| Yellow Island | — | — | — | — | — | — | — | 0 |
| Total per species | 64 | 395 | 34 | 6,234 | 194 | 9,800 | 37 | 16,758 |
| Percent of total population | 0.4 | 2.3 | 0.2 | 37.2 | 1.2 | 58.5 | 0.2 | 100.0 |
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 glaucous-winged gulls (32%) make up 97% of the total San Juan Islands seabird population.
Food supply and availability of nest sites are two critically important factors influencing the distribution and abundance of seabirds. 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 seabird 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 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.