The ecology, morphology, and much of the behavior of a seabird species are definable in terms of the food resources it exploits year-round and the spatial and temporal relations between food and breeding sites. This general point unifies such important reports as those by Kuroda (1954), Bédard (1969a), Ashmole and Ashmole (1967), Ashmole (1971), Spring (1971), and Sealy (1972). More concretely, information on trophic relations of seabirds is useful in several ways. In conjunction with biological oceanographic data, it can provide insight into geographic location, marine habitat, depth, time of day, and general method of food capture by seabirds. Collected over several years, it can provide a basis for understanding annual differences in seabird breeding phenology and success. Finally, supplemented with data on how much seabirds eat and excrete, it is necessary for an understanding of the energetic and ecological roles played by the birds in the functioning of marine ecosystems.

Several studies that describe trophic relations within seabird communities have helped to define the principals of community organization pertaining to the exploitation of available food resources and have given clues to food-chain pathways. Trophic relations have been described for breeding communities in the Barents Sea (Uspenski 1958; Belopol'skii 1961), in the tropical Pacific Ocean (Ashmole and Ashmole 1967; Ashmole 1968), in the North Sea (Pearson 1968), and in the Chukchi Sea (Swartz 1966). The last-named study pertained most directly to the geographic region discussed in this paper, but several other studies have provided sound information on segments of communities in the northeastern North Pacific and Bering Sea. These include the work on three species of auklets (Aethia, Cyclorrhynchus) in the Bering Sea (Bédard 1969a); investigations on cormorants and other fish predators in British Columbia by Munro (1941), Munro and Clemens (1931), and Robertson (1974); studies of murres in Bristol Bay by Ogi and Tsujita (1973); observations on several species near the Pribilof Islands by Preble and McAtee (1923); work on diving species off Oregon by Scott (1973); and studies of murrelets by Sealy (1975).

A review of available reports reveals three obvious gaps in the emphasis placed in seabird food studies. First, few studies have ever considered in detail the trophic relations of seabird communities during the winter or nonbreeding season. Partial exceptions are the works by Cottam (1939) and others on marine diving ducks, species that are seabirds only during the winter, and by several researchers (Munro and Clemens 1931; Munro 1941; Robertson 1974) on seabirds in British Columbia. Divoky (1976) studied diets of pack-ice gulls during the nonbreeding season, but those species are not included in the present analysis because they rarely are found south of the Bering Strait. Second, no study has considered the trophic relationships of an entire seabird community, i.e., not just breeding species but also nonbreeding species. In the rather broad communities considered here, 50-70% or more of the birds breed in another part of the world. To say that these nonbreeding species have no significant impact on resource exploitation or on organization and evolution among breeding members would be naive. Finally, few investigators have attempted to fit birds into an entire ecosystem, including lower trophic level origins as well as fish, marine mammals, and man.

The reasons for these gaps in study emphasis are readily apparent: the inconvenience of marine research during the winter when weather is stormy, the need for costly study platforms (boats), and the difficulties in organizing the specialized community of biologists required for such tasks. A less obvious but important reason is that oceanographers and fishery biologists have overlooked seabirds as important members of marine ecosystems.

Diets of Seabirds in Western North America

Relatively good information exists for most pelecaniformes of the region. A notable exception is the brown pelican (Pelecanus occidentalis), an endangered species. This is unfortunate because dietary information is important for understanding the species' ecology. Observations in eastern North America (Palmer 1962) and Peru (Murphy 1936) indicated that their diet consisted of fish that occur at the surface. The larger cormorants are piscivorous, particularly on schooling fishes that occur at moderate to great depths (Table 1). The smaller cormorants feed more heavily on benthic fish and decapod crustaceans. Cormorants apparently feed only during daylight and then only for short periods because their wettable plumage loses its buoyancy. Thus they remain relatively close (50 km) to nesting and loafing areas.

Table 1. Food of cormorants in different localities (x = major prey, o = minor prey and * = incidental prey species)
Lo­ca­tionDi­et
CRUS­TA­CEANFISH
A
m
p
h
i
p
o
d
I
s
o
p
o
d
D
e
c
a
p
o
d
C
l
u
p
e
a
E
n
g
r
a
u
l
i
s
S
a
l
m
o
n
i
d
A
r
g
e
n
t
i
n
i
d
P
o
r
i
c
h
t
h
y
s
O
t
o
p
h
i
d
i
u
m
B
o
r
e
o
g
a
d
u
s
M
i
c
r
o
g
a
d
u
s
G
a
s
t
e
r
o
s
t
e
u
s
S
e
b
a
s
t
e
s
H
e
x
a
g
r
a
m
m
i
d
C
o
t
t
i
d
A
g
o
n
i
d
E
m
b
i
o
t
o
c
i
d
C
h
r
o
m
i
s
O
x
y
j
u
l
i
s
S
t
i
c
h
a
c
i
d
P
h
o
l
i
d
G
o
b
i
i
d
A
m
m
o
d
y
t
e
s
P
l
e
u
r
o
n
e
c
t
i
d
B
o
t
h
i
d
Double-crested cormorant (Phalacrocorax auritus)[16]
Alaska Peninsula (Palmer 1962)oooxxxox
SE Alaska (Bailey 1927)x
Mandarte Island (Robertson 1974)******oxx
Vancouver Island (Munro and Clemens 1931)x
Oregon (Palmer 1962)x**x*x**
Farallon Island (PRBO, unpublished data)***x***
Brandt's cormorant (P. penicillatus)[17]
Vancouver Island (Robertson, unpublished data)xox
Vancouver Island (Munro and Clemens 1931)x
Washington (Jewett et al. 1953)x
Yaquina Head (Scott 1973)xoo*ooo
Farallon Island (PRBO, unpublished data)*o**xxx*o***
San Diego (Hubbs et al. 1970)**oo*x
Pelagic cormorant (P. pelagicus)[18]
Cape Thompson (Swartz 1966)xxxx
Pribilof Island (Preble and McAtee 1923)xxx
Alaska (Palmer 1962)xxxxxxxxx
SE Alaska (Heath 1915)xxx
Mandarte Island (Robertson 1974)x**xx
Vancouver Island (Munro and Clemens 1921)xxxxxx
Washington (Jewett et al. 1953)xxxx
Netarts, Oregon (Gabrielson and Jewett 1940)**xx
Yaquina Head (Scott 1973)oox
Farallon Island (PRBO, unpublished data)xxxx
Red-faced cormorant (P. urile)
Pribilof Islands (Preble and McAtee 1923)xxxxxx