Conservation of marine birds of northern North America: / papers from the international symposium held at the Seattle Hyatt House

by Unknown

FOOTNOTES:

[5] Data are minimum estimates of pairs and do not include breeding sites with less than 100 birds.

[6] Does not include the black oystercatcher, marbled murrelet, and western gull.

[7] Estimates only for colonies of 100 or more birds.

[8] Estimates are in number of pairs.

[9] Estimates are number of pairs.

[10] Estimates are numbers of pairs.

[11] Data for southeastern Alaska were inadequate to enable estimates of breeding pairs.

[12] Population estimates are minimum and represent numbers of pairs.

[13] Does not include the following species for which population estimates are lacking: black oystercatcher, herring gull, black-legged kittiwake, marbled murrelet, Kittlitz's murrelet.

[14] Data for southeastern Alaska were inadequate to enable estimates of breeding pairs.

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[THE BIOLOGY AND ECOLOGY OF MARINE BIRDS IN THE NORTH]

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[Trophic Relations of Seabirds in the Northeastern Pacific Ocean and Bering Sea]

by

David G. Ainley

Point Reyes Bird Observatory
Stinson Beach, California 94970

and

Gerald A. Sanger[15]

National Marine Fisheries Service
Marine Mammal Division
Seattle, Washington

Abstract

Literature on the diets of seabirds is reviewed for 70 species found in five subarctic oceanographic regions of the northeastern North Pacific Ocean and Bering Sea. Species inhabiting estuaries and sheltered bays are not included. The diets of cormorants, marine ducks, alcids, and marine raptors are best known; less information is available for loons, grebes, petrels, and gulls. Enough is known, however, to broadly characterize the diet of each species. Less than 7% of all species feed on one type of prey, about 60% feed on two or three types, and the rest feed on four or more types. Only 12% of all species feed on eight or more types of prey. Most seabirds (77%) feed as secondary and tertiary carnivores. Where overlap in diet exists, seabirds partition resources through use of different feeding methods, selection of different-sized prey, and zonation of habitat. Species that have specialized diets are probably more susceptible than others to local environmental catastrophes. Species whose feeding methods are highly adapted for exploitation of resources in polar and subpolar habitats are not adapted for coping with oil pollution. Competition between birds and man for marine resources can sometimes benefit seabirds and at other times harm them. More research is needed on seabird feeding relations so that the ecological roles played by marine birds can be defined and placed in perspective. Such work should be conducted at the community level, year-round, and should be so conducted as to facilitate comparison with biological oceanographic data.

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)

Lo­ca­tion	Di­et
	CRUS­TA­CEAN			FISH
	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)	o	o	o	x			x				x									o				x
SE Alaska (Bailey 1927)				x
Mandarte Island (Robertson 1974)			*	*	*	*						*			*		o				x		x
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)				x													o						x
Vancouver Island (Munro and Clemens 1931)				x
Washington (Jewett et al. 1953)																								x
Yaquina Head (Scott 1973)					x	o							o	*	o		o							o
Farallon Island (PRBO, unpublished data)				*	o	*		*	x		x		x	*	o		*							*	*
San Diego (Hubbs et al. 1970)													*		*		o	o						*	x
Pelagic cormorant (P. pelagicus)[18]
Cape Thompson (Swartz 1966)			x							x										x			x
Pribilof Island (Preble and McAtee 1923)			x												x						x
Alaska (Palmer 1962)			x	x							x			x	x	x					x		x	x
SE Alaska (Heath 1915)			x	x											x
Mandarte Island (Robertson 1974)			x												*					*	x		x
Vancouver Island (Munro and Clemens 1921)			x	x									x		x						x	x
Washington (Jewett et al. 1953)				x		x									x									x
Netarts, Oregon (Gabrielson and Jewett 1940)	*	*	x												x
Yaquina Head (Scott 1973)					o	o									x
Farallon Island (PRBO, unpublished data)			x										x		x						x
Red-faced cormorant (P. urile)
Pribilof Islands (Preble and McAtee 1923)			x											x	x						x		x	x

Table 2. Food of marine ducks and geese (x = major prey, o = minor prey, and * = incidental prey species).[19]

Location	Diet[19]
	PLANTS	CRUS­TA­CEANS			MOL­LUSCS						ECHI­NO­DERMS	FISH	FISH EGGS
		Am­phi­pods	Dec­a­pods	Bar­na­cles	Mus­sels	Rock clams	Ra­zor clams	Oys­ters, Scal­lops	Lit­tor­i­nids	Chi­tons
Geese	x
(Branta spp.)
Emperor goose	x
(Philacte canagica)
Oldsquaw	o			*	*	*	*	*	*		*	o
(Clangula hyemalis)
Harlequin duck	*	x	x	o	*			*	o	o	*	*
(Histrionicus histrionicus)
Steller's eider	o	x	*	*	*		*	o	*	*	*	*
(Polysticta stelleri)
Common eider	*	*	x	*	o		*	o	o	*	o	*
(Somateria mollissima)
King eider	*	*	o	*	x		*	o	o	*	x	*
(S. spectabilis)
Spectacled eider	x	*					x	o	*	*	*	*
(S. fischeri)
White-winged scoter	*	*	*	*	o	x	*	x	o	*	*	o	o
(Melanitta deglandi)
Surf scoter	o	*	*	*	x	*	*	x	o	*	*	*	o
(M. perspicillata)
Black scoter	o	*	*	o	x	*	*	x	*	*	*	*
(M. nigra)
Red-breasted merganser												x
(Mergus serrator)

Information on diets of marine ducks (Table 2) is more nearly complete than for most other seabirds. These birds fall into four groups with some overlap: species feeding on plants (Branta, Philacte, Anas-type, and Somateria fischeri); those feeding on benthic crustaceans (Clangula hyemalis, Histrionicus histrionicus, Polysticta stelleri, S. mollissima); those feeding on benthic molluscs (Somateria spp. and Melanitta spp.); and those feeding on fish (Mergus serrator, Clangula hyemalis, and Melanitta deglandi). A study by Perthon (1968), one of the few on a seabird's diet during most of a year, showed a seasonal change in diet for S. mollissima in Norway. In general, waterfowl seem to specialize in their diets much more than other seabirds and, for that reason, are perhaps more restricted in their distributions. Some marine ducks are known to dive to considerable depths (reviewed by Kooyman 1974), but usually they occur in shallow waters where plants and sessile invertebrates are readily available.

The summer diet of the pigeon guillemot (Cepphus columba) is the best known among seabirds in the region being considered here (Table 3). Only in the extreme southern part of its range (i.e., the California Channel Islands) is there no information available on its diet. The species feeds on organisms, mostly fish, from rocky habitat and apparently can dive to considerable depths (Follett and Ainley 1976). Because so much is known about guillemot diets during summer, a study of the winter diet would be valuable.

The diets of other alcids are known well enough to at least characterize them broadly. The larger species, murres, tufted and horned puffins (Lunda cirrhata, Fratercula corniculata), and the rhinoceros auklet (Cerorhinca monocerata), feed heavily on fish, mainly species that school in midwater (Table 4). To a great degree, these birds are opportunistic, feeding rather heavily at times on cephalopods and crustaceans, particularly nektonic forms. Morphological differences between the two murre species suggest that thick-billed murres (Uria lomvia) feed on benthic organisms much more than do common murres (U. aalge), and that the latter species is more piscivorous (Spring 1971); however, field data on diets are barely adequate to confirm this. Ogi and Tsujita (1973) analyzed the stomach contents of murres drowned in salmon gill nets but did not separate the two species. For the present paper we considered them to be mostly U. aalge, since this species predominates in the region of the food study (Bartonek and Gibson 1972). Adult murres sometimes eat different items than they feed to their chicks (Spring 1971; Scott 1973). The smaller alcids, ancient and marbled murrelets—Synthliboramphus antiquus and Brachyramphus marmoratus—(Table 5) and auklets (Table 6), feed on macrozooplankton: crustaceans, and fish and squid larvae. Little is known about the food or feeding ecology of Kittlitz's murrelet (B. brevirostris). Its diet is probably similar to that of the other murrelets, especially the marbled murrelet, its allopatric congener, but the diets of the other murrelets differ somewhat (Bédard 1969b; Sealy 1975). The Kittlitz's murrelet's shorter bill suggests that it feeds more on invertebrates. Alcids feed in deep or shallow water, depending on food distribution. Some alcid species can be found at great distances from land, particularly in winter (Hamilton 1958; Scott et al. 1971).

Information on the diets of other seabirds in the region is fragmentary and sometimes rather anecdotal. A little is known about the feeding habits of loons (Gavia spp.) and grebes (Podiceps spp. and Aechmophorus occidentalis), especially off British Columbia (Table 7). The larger of these birds feed mainly on inshore fish, but as species become progressively smaller, there is a tendency toward eating crustaceans. Work by Madsen (1957) in Denmark, indicated that loons and grebes tend to take prey near or on the bottom. Much more information is available on these birds' diets at their freshwater breeding sites but this provides only partial insight into what they might eat in marine habitats.

Information is especially poor for albatrosses and petrels (order Procellariiformes) (Table 8). Yet, based on sheer numbers alone, members of this diverse group are easily among the most ecologically dominant of the region (Sanger 1972; Ainley 1977). The Laysan albatross (Diomedea immutabilis) seems to be a squid specialist; the black-footed albatross (D. nigripes), northern fulmar (Fulmarus glacialis), scaled petrel (Pterodroma inexpectata), and the fork-tailed and Leach's storm-petrels (Oceanodroma furcata and O. leucorhoa) appear to be large, medium, small, and tiny versions, respectively, of surface-feeding generalists that eat whatever they can find, including live and dead fish, squid, coelenterates, crustaceans, and other organisms. The shearwaters (Puffinus spp.) feed to an unknown degree on schooling fish, squid, and crustaceans that occur near the surface. For these very abundant shearwaters, that, unfortunately, is close to the extent of our knowledge both for the North Pacific, where they winter, and the South Pacific, where they breed. Most petrels remain in oceanic habitats, but shearwaters, particularly the sooty shearwater (Puffinus griseus), and sometimes fulmars feed close to, if not within, the inshore neritic habitat. A much better understanding of the diets of this group is sorely needed.

Table 3. Food of the pigeon guillemot (Cepphus columba) in different localities (x = major prey, o = minor prey, and * = incidental prey species).

Lo­ca­tion	Di­et
	CRUS­TA­CEAN			O C T O P U S	FISH
	A m p h i p o d	I s o p o d	D e c a p o d		P e t r o m y z o n t i d	C h i m a e r i d	C l u p e i d	O s m e r i d	G a d i d	G a s t e r o s t e i d	S c o r p a e n i d	C o t t i d	A g o n i d	E m b i o t o c i d	B a t h y m a s t e r i d	C l i n i d	C r y p t a c a n t h o d i d	C e b i d i c h t h y i d	S t i c h a e i d	P h o l i d	A m m o n d y t i d	B o t h i d	P l e u r o n e c t i d
Cape Thompson (Swartz 1966)									o										o
Pribilof Island (Preble and McAttee 1923)	o	o	o
Mandarte Island (Drent 1965; Koelink 1972)			o		o		*			o		x	o	o	*	o	*	o	o	o	o	o	o
Vancouver Island (Munro and Clemens 1931)			o																			o	o
Olympic Peninsula (Thoresen and Booth 1958)					o			o												o	o
Yaquina Head (Scott 1973)			o																o	o		o	o
Farallon Island (Follett and Ainley 1976)			o	o		*					x	x	*						o	o		o	o

Knowledge on the food of gulls, shorebirds, and related species is surprisingly scanty in view of all that is known about their breeding biology and social behavior. Little is known about the marine food of phalaropes, but by inference from their association with storm-petrels, plankton-feeding whales, and convergence lines (Martin and Myers 1969), their tiny size, and their method of feeding (picking at minuscule items on the water surface), one can guess that they feed on zooplankton and detritus. Skuas (Catharacta skua) and jaegers (Stercorarius spp.) apparently eat what they can find at the surface, as well as whatever they can steal from gulls and terns. Almost all the literature on their feeding (Bent 1946) dwells on accounts of their stealing from other birds. That spectacular behavior would seem to be so energetically costly, though, that it is probably less important than we have been led to believe. Rather surprisingly, the question of what foods the gulls and terns eat in the eastern North Pacific is difficult to answer from the literature (Tables 9 and 10). Some information exists for five of the larger larids at isolated places, but little is known about food elsewhere in their respective ranges, and the diets of the seven smaller gulls and the terns are practically unknown. Studies on gull diets in the Atlantic region (e.g., Spaans 1971; Harris 1965) provide information on what to expect from the same species in the Pacific, but that information must be considered only in general terms because, the birds being somewhat opportunistic, their diets differ greatly from one locality to another (Ingolfsson 1967). A few observations are available for arctic terns (Sterna paradisaea) in Alaska, but little information exists for other terns (Table 10). Bent (1921) noted that Aleutian terns (S. aleutica) sometimes associate with arctic terns during feeding.

Finally, we must include raptors, particularly the peregrine (Falco peregrinus) and bald eagle (Haliaeetus leucocephalus), because they are important predators on the smaller seabirds (White et al. 1971, 1973). Peregrines have, in fact, been observed feeding on storm-petrels far at sea (Craddock and Carlson 1970).

Trophic Relations Within Seabird Communities

We have compared and summarized in general terms the food partitioning by species in five rather broad oceanographic regions and their subdivisions in the northeastern North Pacific and Bering Sea, based on the specific details on diets presented in Tables 1 through 10. The five broad regions, defined oceanographically by Dodimead et al. (1963) and Favorite et al. (1976) and modified by Sanger (1972), are shown in Fig. 1. The five oceanographic regions (domains) were divided further into inshore neritic, offshore neritic, and oceanic habitats (Sanger and King, this volume). We did not include estuarine habitats or sheltered bays in the analysis.

Fig. 1. Schematic oceanographic domains of the subarctic Pacific regions (defined by Dodimead et al. (1963) and Favorite et al. (1976) and modified by Sanger (1972).)

Table 4. Food of murres and puffins in different localities (x = major prey, o = minor prey, and * = incidental prey species).

LOCATION	Diet
	CRUS­TA­CEAN				P O L Y C H A E T E	C E P H A L O P O D	FISH
	E u p h a u s i i d	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	S a r d i n o p s	E n g r a u l i s	S a l m o	O n c h o r h y n c h u s	H y p o m e s u s	T h a l e i c h t h y s	M a l l o t u s	B o r e o g a d u s	M i c r o g a d u s	T h e r a g r a	L y c o d e s	G a s t e r o s t e u s	S e b a s t e	T r i g l o p s	M y o x o c e p h a l u s	C o t t i d	C y m a t o g a s t e r	E m b i o t o c i d	C h i r o l o p h i s	S t i c h a e i d	A m m o d y t e s	P l e u r o n e c t i d	L i p a r i d
Common murre(Uria aalge)[20]
Cape Thompson (Swartz 1966)	o	o	o		o	o									x						o	o				o		x
Pribilof Islands (Preble and McAtee 1923)		x	o		o																		o
E. Bering Sea (Ogi and Tsujita 1973)	x					o								x			x											x
Forrester Island (Heath 1915)	o	o																										x
Vancouver Island				o		o	x																	o
(Robertson, unpublished data)									x			x								o								o
Olympic Peninsula (Cody 1973)									x			x	o							x				o				o	o
Yaquina Head	x				o
(Scott 1973)									x	o		o				*				x				o					o
Farallon Islands (PRBO, unpublished data)	x				o
Thick-billed murre (U. lomvia)[21]															x			o			o	o				o	o	x	o
Cape Thompson (Swartz 1966)		o		o	o
Pribilof Islands (Preble and McAtee 1923)				x		x
Hooker Island	x				o
(Demme 1934, in Dement'ev et al. 1968)														o	x					*	x	*	o				x	o	o	o
NE Canada (Tuck and Squires 1937)		x		o	o	o
Tufted puffin															x
(Lunda cirrhata)[22]																							*
Cape Thompson						*
(Swartz 1966)																												x
Forrester Island (Heath 1915)																												x
Langara Island																	x
(Sealy 1973a)							x	x				x													x
Washington																				o
(Jewett et al. 1953)									x			o																x
Olympic Peninsula																				x
(Cody 1973)									x			o
Farallon Island (PRBO, unpublished data)						x
Horned puffin															x						o
(Fratercula corniculata)														o														x
Cape Thompson (Swartz 1966)					o
Alaska (Bent 1946)																												x
Forrester Island (Heath 1915)
Rhinoceros auklet (Cerorhinca monocerata)							x				o	x		x
NW Pacific (Kozlova 1961;	x
Komaki 1967)																												x
Forrester Island (Heath 1915)																												x
Langara Island (Sealy 1973a)												o																x
Destruction Island																			o
(Richardson 1961)									x			x																o
Olympic Peninsula (Cody 1973)								x
So. California (Linton 1908; Grinnell 1899)	x

Table 5. Food of ancient and marbled murrelets (x = major prey, o = minor prey, and * = incidental prey species).

Location	DIET
	CRUSTACEAN										SQU­ID	FISH
	Eu­phau­si­id	Thy­sa­noes­sa	Eu­phau­sia	My­sid	A­can­tho­my­sis	Am­phi­pod	Gam­ma­rid	Ca­rid shrimp	Dec­a­pod	Lar­vae	Lar­vae	En­gra­u­lis	Os­mer­id	Scor­paen­id	Cy­ma­to­ga­ster	Sti­chae­id	Am­mo­dy­tes	Lar­vae
Ancient murrelet (Synthliboramphus antiquus)[23]
Commander Islands (Dement'ev et al. 1968)						x		x
Amchitka Island (White et al. 1971, 1973)	x	x		x	x												x
Langara Island (Sealy 1975)	x	x	x			*			*	*				o	o		x
Marbled murrelet (Brachyramphus marmoratus)[24]
SE Alaska (Grinnell 1897)											o
Langara Island (Sealy 1975)	x	x						*			*		*	*	x	*
Vancouver Island (Munro and Clemens 1931)			x												x			x
Olympic Peninsula (Cody 1973)												x					x

Table 6. Diets of auklets in different localities (x = major prey, o = minor prey, and * = incidental prey species).

Location	Diet
	CRUSTACEAN											PO­LY­CHAE­TE	SQUID	FISH
	Eu­pha­u­si­id	Thy­sa­noes­sa	My­sid	Sty­lo­my­sis	Am­phi­pod	Pa­ra­the­mi­sto	Phro­ne­ma	Gam­ma­rid	Co­pe­pod	Ca­la­nus	Ca­rid shrimp		Lar­vae	Cot­tid	Lar­vae
Cassin's auklet (Ptychoramphus aleuticus)
Forrester Island (Heath 1915)	x				x				x
Olympic Peninsula (Cody 1973)	x														x
Farallon Islands (Manuwal 1974)	x	x			x		x		x				o		x
Parakeet auklet (Cyclorrhynchus psittaculus)
Chukhotsk Peninsula (Portenko 1934, in Dement'ev et al. 1968)									x	x		x x		x
Aleutian Islands (Bent 1946)					x
St. Lawrence Island (Bédard 1969a)	x	x	o		x	x		o	o	*	o	o	o
Crested auklet (Aethia cristatella)[25]
W. Bering Sea (Portenko 1934, in Dement'ev et al. 1968)					x							x
Commander Islands (Stejneger 1885)													x
Amchitka (White et al. 1973)	x	x	x	x									x
St. Lawrence Island. (Bédard 1969a)				x	x	x		o o	x	x	*
Pribilof Islands (Preble and McAtee 1923)					x	x
Least auklet (A. pusilla)
Commander Islands (Stejneger 1885)					x
Aleutian Islands (Bent 1946)					x
St. Lawrence Island (Bédard 1969a)			o	o	*	o	o	o	x	x	x
Whiskered auklet (A. pygmaea)
Commander Islands (Stejneger 1885)	x			x			*

Table 7. Diets of loons and grebes in different localities (x = major prey, o = minor prey, and * = incidental prey species).

Location	Diet
	CRUSTACEANS				PO­LY­CHAE­TE	FISH
	Eu­pha­us­id	Am­phi­pod	My­sid	De­ca­pod		An­gu­il­la	Cla­pea	Sar­di­nops	Sal­mo	Tha­le­ich­thys	A­the­ri­nops	Zo­ar­chid	Ga­did	Fun­du­lus	Ga­ste­ro­ste­us	Se­ba­stes	Cat­tid	Cy­ma­to­ga­ster	Sti­chae­id	Am­mo­dy­tes	Go­bi­id
Common loon (Gavia immer)
Alaska (Palmer 1962)		*		*			o		o							o	o	o
Vancouver Island (Munro and Clemens 1931)							x
Denmark (Madsen 1957)						*						o	x		*		o
Yellow-billed loon (G. adamsii)[26]
Alaska (Cottam and Knappen 1939)				*	*								o				x
Alaska (Bailey 1922)																x
Arctic loon (G. arctica)[27]
Vancouver Island (Palmer 1962)							x
Vancouver Island (Robertson, unpublished data)							x
California (Palmer 1962)																		x
Denmark (Madsen 1957)						*	o					*	x		x		*			x	*
Red-throated loon (G. stellata)[28]
Oregon (Palmer 1962)																	x
No. Atlantic (Palmer 1962)													x				x			o
Denmark (Madsen 1957)						*	o					*	x		o		*			*	o
Western grebe (Aechmophorus occidentalis)
Vancouver Island (Munro 1941)				o	*		x										x
Vancouver Island (Robertson, unpublished data)							x											x
Puget Sound (Phillips and Carter 1957)							x						*				o	o	o
Washington (Chatwin 1956)						*				x
California (Palmer 1962)				o	*		x				x						x		*
Red-necked grebe (Podiceps grisegena)
Pribilof Islands (Preble and McAtee 1923)		o
Vancouver Island (Wetmore 1924)															x
Vancouver Island (Munro 1941)							x	o							x
E. No. America (Wetmore 1924)				o	*	o								o			x
Horned grebe (P. auritus)[29]
Pribilof Islands (Preble and McAtee 1923)		x			o
W. No. America (Wetmore 1924)		x	x		*										o		o
Vancouver Island (Munro 1941)	x			x												o	o	*
Denmark (Madsen 1957)		o	o		o																o
Eared grebe (P. nigricollis)[30]
W. No. America (Wetmore 1924)		*	x		*												o
Vancouver Island (Munro 1941)		x	x			o
Denmark (Madsen 1957)			x				*														o

Table 8. Diets of albatrosses and petrels in different localities (x = major prey, o = minor prey, and * = incidental prey species).

Location	Diet
	CRUSTACEAN						COE­LEN­TE­RA­TE	E­CHI­NO­DERM	CE­PHA­LO­POD	FISH
	Eu­pha­u­si­id	Am­phi­pod	Co­pe­pod	De­ca­pod	Lar­vae	Bar­na­cle				"Fish"	En­grau­lis	My­cto­phid	Se­ba­stes	Am­mo­dy­tes	Car­ri­on, fish of­fal	Fish eggs
Black-footed albatross (Diomedea nigripes)
No. Pacific (Palmer 1962)									x	x					x
Aleutian Islands (Cottam and Knappen 1939)		o		x				o					x		x
California (Miller 1936, 1940)				o		o			x	x					x	x
Laysan albatross (D. immutabilis)
No. Pacific (Palmer 1962; Bartsch 1922; Fisher 1904)									x
Northern fulmar (Fulmarus glacialis)
Pribilof Islands (Preble and McAtee 1923)							o		x
Alaska (Gabrielson and Lincoln 1959)							x		x						x
Oregon (Gabrielson and Jewett 1940)									x
No. Atlantic (Hartley and Fisher 1936; Einarsson 1945; Fisher 1952)	x														x
Flesh-footed shearwater (Puffinus carneipes)
Australia (Oliver 1955; Serventy et al. 1971)	x								x	x
Pink-footed shearwater (P. creatopus)
California (Murphy 1936; Ainley, personal observation)									x	x
E. Pacific (Cottam and Knappen 1939)									x	x
Buller's shearwater (P. bulleri)
SW Pacific (Falla 1934; Serventy et al. 1971)	x								x	x
Peru (Murphy 1936)	x
Sooty shearwater (P. griseus)
Aleutian Islands (Sanger, personal observation)	x								x			x
British Columbia (Martin 1942; Sealy 1973a)	x										x				x
Oregon (Gabrielson and Jewett 1940)									x
California (Ainley, personal observation)									x		x
Peru (Murphy 1936)				x					x		x
SW Pacific (Oliver 1955; Serventy et al. 1971)	x								x		x
Short-tailed shearwater (P. tenuirostris)
Bristol Bay (Bartonek, personal communication)	x
Alaska (Cottam and Knappen 1939)	x	x							x	o
No. Pacific (Palmer 1962; Kuroda 1955)	x								x	x
Australia (Serventy et al. 1971)	x								x		x
Bass Strait (Sheard 1953)	x
Mottled petrel (Pterodroma inexpectata)
Pacific Ocean (Imber 1973)										x
E. No. Pacific (Kuroda 1955)															x
Fork-tailed storm-petrel (Oceanodroma furcata)
Pribilof Islands (Preble and McAtee 1923)										x
SE Alaska (Heath 1915)	x
British Columbia (Martin 1942)															x
California (Ainley, personal observation)										x
Leach's storm-petrel (O. leucorhoa)
SE Alaska (Heath 1915)	x
California (PRBO, unpublished data)	x					x			x		x				x
So. California (Palmer 1962)					x											x
No. Atlantic[31] (Palmer 1962)	x		x						x	x					x

Table 9. Diets of gulls in different localities (x = major prey, o = minor prey, and * = incidental prey species).

Location	Diet
	CRUSTACEAN			P O L Y C H A E T E	MOLLUSC		E C H I N O D E R M	C O E L E N T E R A T E	FISH																C A R R I O N - O F F A L	BIRD			F I S H E G G S
	E u p h a u s i i d	B a r n a c l e	D e c a p o d		S h e l l f i s h	C e p h a l o p o d			" F i s h "	C l u p e a	E n g r a u l i s	O s m e r u s	P o r i c h i h y s	O t a p h i d i u m	M a l l o t u s	B o r r o g a d u s	M i c r o g a d u s	G a d u s	L y c o d e s	S e b a s t e s	M y x o c e p h a l u s	G e n y o n e m u s	E m b i o t o c i d	A m m o d y t e s		E g g s	C h i c k s	A d u l t s
Glaucous gull (Larus hyperboreus)
St. Lawrence Island (Fay and Cade 1959)																										x	x	x
Chukchi Sea (Swartz 1966)			x				x	x													x			x	x	x
Pribilof Islands (Preble and McAtee 1923)			x				x	x	x																x
Vancouver Island (Munro and Clemens 1931)										x																			x
Glaucous-winged gull (L. glaucescens)[32]
Pribilof Islands (Preble and McAtee 1923)			x		x		x																		x	x	x	x
Alaska (Bent 1921)																									x
No. Pacific (Sanger 1973)		x	*		*
Mandarte Island (Ward 1973)			x		x					x														x	x
Vancouver Island (Munro and Clemens 1931; Robertson, unpublished data)			x							x															x
Western gull (L. occidentalis)[33]
Farallon Islands (PRBO, unpublished data)	x	x	o		*	x	*	*			x		x	o			o	o		x		o	o		x	o	o	o
Herring gull (L. argentatus)
No. Atlantic (Zelikman 1961)	x
E. No. America (Bent 1946; Ainley, personal observation)			x	x	x	x			x																x	*		o	x
Vancouver Island (Munro and Clemens 1931)	x								x																x
Mew gull (L. canus)
Alaska (Bent 1921)							x								x										x				x
Vancouver Island (Munro and Clemens 1931)		x			x																								x
Heermann's gull (L. heermanni)
California (Bent 1921)	x				x				x																x
Bonaparte's gull (L. philadelphia)
E. No. America (Bent 1921)	x			x					x
Black-legged kittiwake (Rissa tridactyla)[34]
Chukchi Sea (Swartz 1966)			o	*								o			o	x			o		o			x
Pribilof Islands (Preble and McAtee 1923)			o						x																o
Alaska (Bent 1921)									x																o
Cook Inlet[35] (Snarski, personal communication)			o	o	o										x		o							x
No. Atlantic (Hartley and Fisher 1936; Zelikman 1961)	x
Red-legged kittiwake (R. breuirostris)
Pribilof Islands (Preble and McAtee 1923)	x			x	x
Sabine's gull (Xenia sabini)
Pt. Barrow (Banner 1954)	x

Table 10. Diets of terns in different localities (x = major prey species).

Location	Diet
	CRUSTACEAN			FISH
	"Crustacean"	Mallotus	Euphausiid	Cottid	Amphipod	Ammodytes larvae
Arctic tern (Sterna paradisaea)
Pribilof Islands (Preble and McAtee 1923)			x		x
Alaska (Bent 1921)	x			x		x
No. Atlantic (Hartley and Fisher 1936)		x
Common tern (S. hirundo)
E. No. America (Bent 1921)		x				x

The oceanic habitat includes waters of the photic zone overlying the deep ocean and continental slopes beyond the continental or insular shelves. The Bering Sea and central subarctic domains are largely made up of oceanic habitat. The other three domains include both inshore and offshore neritic as well as some oceanic habitat. The boundary between the inshore and offshore neritic has yet to be defined in terms of bird life, but it lies at that line beyond which the bottom is too deep for a diving bird to exploit. A depth contour thus defines the boundary. In the antarctic South Pacific, emperor penguins (Aptenodytes fosteri) dive to depths of 275 m, but so far as is known, no comparable bird exists in the North Pacific. Some marine ducks and loons reportedly dive to 50-60 m (Kooyman 1974). The inshore-offshore neritic boundary for seabirds may lie near the 70-m depth contour.

Food resource partitioning by seabirds in the five oceanographic domains are shown in Tables 11-15. Within each domain, the common and usual members of the seabird community are listed, and the major and minor categories in each of their diets are shown (on the basis of available literature, Tables 1-10). The categories are grouped further, and rather tenuously, according to the trophic level at which a bird is presumably feeding: I = herbivore, II = secondary carnivore, III = tertiary carnivore, IV = final carnivore, and Sc = scavengers (carnivorous) feeding at many levels. Birds at level I feed on large algae and seed plants and are not directly part of the same food webs involving other species. These food webs originate with phytoplankton (Fig. 2). So far as is known, no bird feeds on phytoplankton and few, if any, feed on microzooplankton; hence birds do not generally feed as primary carnivores. An exception at times might be the least auklet (Aethia pusilla) when it feeds on small copepods (see Bédard 1969b).

The above groupings are "tenuous" because prey in each category may represent more than one trophic level, and a single prey species could occur at one level one day or place and at another level the next day or place, depending upon what it happened to be eating. This is shown in Fig. 2, where the parakeet auklet (Cyclorrhynchus psittaculus) can occur in the food web at different levels, depending both on the prey it is eating and on what its prey is eating. Even without this complication, many seabirds feed at more than one level in the food web. For instance, murres eating euphausiids would be feeding at a different level than murres feeding on larger fish. It might be "safer" to regard prey organisms in level II as macrozooplankton, prey organisms in level III as micronekton, and prey organisms (seabirds themselves) in level IV as macronekton (after Sverdrup et al. 1942).

Fig. 2. Schematic food web of the parakeet auklet in the eastern Bering Sea (based on Bédard 1969a and Dunbar 1946). Arrow sizes indicate relative importance of prey and Roman numerals refer to prey sizes (see text).

Table 11. Use of food resources by seabirds in the Bering Sea coastal domain. Information is from Tables 1-10. (Trophic level I = plants, II = secondary carnivore, III = tertiary carnivore, IV = upper level carnivore [on birds only in this table], Sc = scavenger on carrion, offal, or detritus [II-IV]; x = major food in diet, o = minor food, * = incidental food, ? = probable food.)

Seabirds	Habitat, bird trophic levels (I-IV. Sc), and food categories
	Oceanic and offshore neritic									Inshore neritic
	II				III		IV	Sc	I	II						III			IV	Sc
	C r u s t a c e a n	P o l y c h a e t e	C o e l e n t e r a t e	F i s h / s q u i d e g g s & l a r v a e	F i s h	C e p h a l o p o d	B i r d s	C a r r i o n / o f f a l / d e t r i t u s	P l a n t	C r u s t a c e a n , m i d w a t e r	C r u s t a c e a n , b e n t h i c	C o e l e n t e r a t e	E c h i n o d e r m	M o l l u s c	F i s h / s q u i d e g g s & l a r v a e	F i s h , m i d w a t e r	F i s h , b e n t h i c	C e p h a l o p o d	B i r d s	C a r r i o n / o f f a l / d e t r i t u s
Gavia adamsii											*					o	x
G. arctica																o	x
Podiceps grisegena											o					o	x
Diomedea nigripes	x	o	o	o	x	x		x
Fulmarus glacialis	x	o	x	o	x	x		x
Puffinus griseus	x				x	x										o		o
P. tenuirostris	x				o	x										o		o
Oceanodroma furcata	x	o	o	x	x	x		x
Phalacrocorax auritus											o					x	o
P. pelagicus											x						x
P. urile											x						x
Branta bernicla									x
Philacte canagica									x
Clangula hyemalis										o	x			o			o
Histrionicus histrionicus										o	x			o
Polysticta stelleri									o	x	o			o
Samateria mollissima											x		o	x
S. spectabilis									o		o		o	x
S. fischeri									x					x
Melanitta deglandi														x	o
M. nigra									o		o			x
Haliaeetus leucocephalus																x			x	x
Falco peregrinus																			x
Phalaropus fulicarius	x			x				o		x					x
Lobipes lobatus	x			x				o		x					x
Stercorarius spp.	o				x	x	?	x								x			x	x
Larus hyperboreus	o			o	o	o			o		o	o	o	o	o	x			x	x
L. glaucescens	o			o	o	o			o		o	o	o	o	o	x			x	x
L. argentatus	o			o	o	o			o		o	o	o	o	o	x			o	x
L. canus											x	o	o	o	x					x
Rissa tridactyla	x				x	x			o
Xema sabini	x			x	o	o
Sterna paradisaea	x			x	o	o
Uria aalge	x	o			x	x				o	o					x	o
U. lomvia	x	o			x	x				o	x					o	x
Lunda cirrhata	?				x	x
Fratercula corniculata		*			x	x
Cepphus columba											o						x	o
Synthliboramphus antiquus	x				x					o
Brachyramphus brevirostris										x						o
Cyclorrhynchus psittaculus	x	o		*	*
Aethia cristatella	x			x
A. pusilla	x			o

Table 12. Use of food resources by seabirds in the oceanic and offshore neritic habitats, Bering Sea domain. Information is from Tables 1-10. (Trophic level I = plants, II = secondary carnivore, III = tertiary carnivore, IV = upper level carnivore; Sc = scavenger on carrion, offal, or detritus [II-IV]; x = major food in diet, o = minor food, * = incidental food, ? = probable food.)

Seabirds	Bird trophic levels and food categories
	II				III		IV	Sc
	Cru­sta­ce­an	Po­ly­chae­te	Coe­len­te­ra­te	Fish/Squid eggs & Lar­vae	Fish	Ce­pha­lo­pod	Birds	Car­ri­on/of­fal/de­tri­tus
Diomedea nigripes	x	o	o	o	x	x		x
D. immutabilis						x
Fulmarus glacialis	x	o	x	o	x	x		x
Puffinus griseus	x				x	x
P. tenuirostris	x				o	x
Pterodroma inexpectata					x			x
Oceanodroma furcata	x	o	o	x	x	x		x
Phalaropus fulicarius	x			x				o
Lobipes lobatus	x			x				o
Stercorarius spp.	o				x	x	?	x
Larus hyperboreus	x	o	o	o	x	x	?	x
L. glaucescens	x	o	o	o	x	x	?	x
Rissa tridactyla	x				x	x		o
R. brevirostris	x				x	x		o
Xema sabini	x			x	o	o
Sterna paradisaea	x			x	o	o
Uria aalge	x	o			x	x
U. lomvia	x	o			x	x
Lunda cirrhata	?				x	x
Fratercula corniculata		*			x	x
Synthliboramphus antiquus	x				x
Cyclorrhynchus psittaculus	x	o		*	*
Aethia cristatella	x			x
A. pusilla	x			o
A. pygmaea	x

Information contained in Tables 11-15 can be summarized to show characteristics of seabird trophic relations. One such characteristic is the range of diet breadth or diet complexity (Table 16). Few species (about 6%) feed on only one type of prey and might, therefore, be referred to as "specialists." Included are eared grebe (Podiceps caspicus), Laysan albatross, brown pelican, emperor goose (Philacte canagica), black brant (Bernicia bernicla), peregrine falcon, and whiskered auklet (Aethia pygmaea). Consideration of these species as specialists may require revision when more data become available. Except for the albatross and auklet, these species are members of the inshore neritic cohort. Food specialization does not seem to be characteristic of oceanic birds in particular or of most seabirds in general.

Table 13. Use of food resources by seabirds in the Alaska Stream domain. Information is from Tables 1-10. (Trophic level I = plants, II = secondary carnivore, III = tertiary carnivore, IV = upper level carnivore, Sc = scavenger on carrion, offal, or detritus [II-IV]; x = major food in diet, o = minor food, * = incidental food, ? = probable food.)

Seabirds	Habitat, bird trophic levels (I-IV. Sc), and food categories
	Oceanic and offshore neritic									Inshore neritic Inshore neritic
	II				III		IV	Sc	I	II						III			IV	Sc
	C r u s t a c e a n	P o l y c h a e t e	C o e l e n t e r a t e	F i s h / s q u i d e g g s & l a r v a e	F i s h	C e p h a l o p o d	B i r d s	C a r r i o n / o f f a l / d e t r i t u s	P l a n t	C r u s t a c e a n , m i d w a t e r	C r u s t a c e a n , b e n t h i c	C o e l e n t e r a t e	E c h i n o d e r m	M o l l u s c	F i s h , m i d w a t e r	F i s h , b e n t h i c	C e p h a l o p o d	B i r d s	F i s h / s q u i d e g g s & l a r v a e	C a r r i o n / o f f a l / d e t r i t u s
Gavia immer										*					x	x
G. adamsii										*					o	x
G. stellata										*					o	x
Podiceps grisegena										o					x	o
Diomedea nigripes	x	o	o	o	x	x		x
Fulmarus glacialis	x	o	x	o	x	x		x
Puffinus griseus	x				x	x									o
P. tenuirostris	x				o	x									o
Pterodroma inexpectata					x			x
Oceanodroma furcata	x	o	o	x	x	x		x
Phalacrocorax auritus										o					x	o
P. pelagicus										x						x
P. urile										x						x
Philacte canagica								x
Clangula hyemalis									o	x			o
Histrionicus histrionicus									x	x			o
Polysticta stelleri								o	x	o			o
Somateria mollissima										x		o	x
S. spectabilis										o		o	x
S. fischeri								x					x
Melanitta deglandi													x	o
M. perspicillata								o					x	o
M. nigra								o		o			x
Mergus serrator										o					x	x
Haliaeetus leucocephalus																x			x	x
Falco peregrinus																		x
Phalaropus fulicarius	x			x				o	x								x
Lobipes lobatus	x			x				o	x								x
Stercorarius spp.	o				x	x	?	x								x			x	x
Larus hyperboreus	o			o	o	o		o			o	o	o	o	o	x			x	x
L. glaucescens	o			o	o	o		o			o	o	o	o	o	x			x	x
L. argentatus	o			o	o	o		o			o	o	o	o	o	x			o	x
L. canus											x	o	o	o	x					x
Rissa tridactyla	x				x	x		o
R. brevirostris	x				x	x		o
Sterna paradisaea	x			x	o	o				x						o
S. aleutica										x						o
Uria aalge	x	o			x	x				o	o					x	o
U. lomvia	x	o			x	x				o	x					o	x
Lunda cirrhata	?				x	x
Fratercula corniculata		*			x	x
Cepphus columba											o						x	o
Brachyramphus marmoratus										x					o	x
B. brevirostris										x						o
Synthliboramphus antiquus	x				x					x
Cyclorrhynchus psittaculus	x	o		*	*
Aethia cristatella	x			x
A. pusilla	x			o
A. pygmaea	x

Table 14. Use of food resources by seabirds in the oceanic habitat, central subarctic domain. Information is from Tables 1-10. (Trophic level I = plants, II = secondary carnivore, III = tertiary carnivore, IV = upper level carnivore, Sc = scavenger on carrion, offal, or detritus [II-IV]; x = major food in diet, o = minor food, * = incidental food, ? = probable food.)

Seabirds	Bird trophic levels and food categories
	II				III		IV	Sc
	Crustacean	Polychaete	Coelenterate	Fish/squid eggs & larvae	Fish	Cephalopod	Birds	Carrion/offal/detritus
Diomedea nigripes	x	o	o	o	x	x		x
D. immutabilis						x
Fulmarus glacialis	x	o	x	o	x	x		x
Puffinus carneipes	o				x	x
P. griseus	x				x	x
P. tenuirostris	x				o	x
Pterodroma inexpectata					x			x
Oceanodroma furcata	x	o	o	o	x	x		x
O. leucorhoa	x	o	o	o	x	x		x
Phalaropus fulicarius	x			x				o
Lobipes lobatus	x			x				o
Stercorarius spp.	o				x	x	?	x
Larus hyperboreus	x	o	o	o	x	x	?	x
L. glaucescens	x	o	o	o	x	x	?	x
L. argentatus	x	o	o	o	x	x		x
Rissa tridactyla	x				x	x		o
Xema sabini	x			x	o	o
Sterna paradisaea	x			x	o	o
Uria aalge	x	*			x	x
U. lomvia	x	*			x	x
Lunda cirrhata	o				x	x
Fratercula corniculata		*			x	x
Cerorhinca monocerata	x				x
Synthliboramphus antiquus	x				x
Cyclorrhynchus psittaculus	x	o		*	*
Ptychoramphus aleuticus	x			o

Most species (roughly 53% in any community) include two or three prey categories in their diets—usually midwater schooling fish, squid, and crustaceans. These birds include the most numerous in the communities—the shearwaters and some alcids—which feed largely on three prey types, and also include some of the less abundant birds, the marine ducks, which feed mostly on two prey categories.

The remaining seabirds are more general in their feeding. Many have large populations, but are not as abundant as shearwaters or most alcids. The true "generalists" are the species that feed on as many as eight or more types of prey, and relatively few (12%) such species exist in each avian community. These birds, the scavengers, include black-footed albatross, fulmar, storm-petrels, and large gulls. The petrels are the scavengers of the oceanic habitat and the gulls are their counterparts in the neritic habitat (but see Sanger 1973).

Another comparison is shown in Table 17, where the species in each community are categorized according to the number feeding at each trophic level. If a species feeds at more than one level, it is tallied once in each level. Most seabirds (66-77%) feed at the second and third levels as secondary and tertiary carnivores. Few feed as terminal carnivores, and relatively few are scavengers. Actually, most scavenging occurs at levels II and III, so about 90% of the seabirds in each community feed at levels II and III. Communities including an inshore neritic feeding element are the only ones that include herbivores, and even then, few of these species exist in significant numbers in the marine environment (discounting estuaries and sheltered bays).

Table 15. Use of food resources by seabirds in the North American coastal domain. Information is from Tables 1-10. (Trophic level I = plants, II = secondary carnivore, III = tertiary carnivore, IV = upper level carnivore, Sc = scavenger on carrion, offal, or detritus [II-IV]; x = major food in diet, o = minor food, * = incidental food, ? = probable food.)

Seabirds	Habitat, bird trophic levels (I-IV, Sc), and food categories
	Oceanic and offshore neritic									Inshore neritic
	II				III		IV	Sc	I	II						III			IV	Sc
	C r u s t a c e a n	P o l y c h a e t e	C o e l e n t e r a t e	F i s h - s q u i d e g g s & l a r v a e	F i s h	C e p h a l o p o d	B i r d s	C a r r i o n / o f f a l / d e t r i t u s	P l a n t	C r u s t a c e a n , m i d w a t e r	C r u s t a c e a n , b e n t h i c	C o e l e n t e r a t e	E c h i n o d e r m	M o l l u s c	F i s h / s q u i d e g g s & l a r v a e	F i s h , m i d w a t e r	F i s h , b e n t h i c	C e p h a l o p o d	B i r d s	C a r r i o n / o f f a l / d e t r i t u s
Gavia immer											*					x	x
G. adamsii											*					o	x
G. arctica																o	x
G. stellata																o	x
Podiceps grisegena											o					x	o
P. nigricollis											x						o
P. auritus										x	x						o
Aechmophorus occidentalis											o					x	x
Diomedea nigripes	x	o	o	o	x	x		x
Fulmarus glacialis	x	o	x	o	x	x		x
Puffinus creatopus	o				x	x
P. carneipes	o				x	x
P. bulleri	x				x	x
P. griseus	x				x	x				o						o	o
P. tenuirostris	x				o	x				o						o	o
Oceanodroma furcata	x	o	o	o	x	x		x
Pelecanus occidentalis																x
Phalacrocorax auritus											o					x	o
P. penicillatus																o	x
P. pelagicus											x						x
Branta bernicla								x
Clangula hyemalis										o	x			o			o
Histrionicus histrionicus										o	x			o
Melanitta deglandi														x	o		o
M. perspicillata									o					x	o
M. nigra				*					o		o			x
Mergus serrator											*					x	x
Haliaeetus leucocephalus																x			x	x
Falco peregrinus																			x
Phalaropus fulicarius	x			x				o		x					x					o
Lobipes lobatus	x			x				o		x					x					o
Stercorarius spp.	o				x	x	?	x								x	x		o	x
Larus hyperboreus	o			o	o	o	?	o			o	o	o	o	o	x			x	x
L. glaucescens	o			o	o	o	?	o			o	o	o	o	x	x			x	x
L. occidentalis	x			x	x	x		o		x	o	o	o	*	*	x		x	o	x
L. argentatus	o			o	o	o		o		x	o	o	o	o	o	x			o	x
L. heermanni										x						x				x
L. canus											x	o	o	o	x	x				x
L. philadelphia										x					x	o
Rissa tridactyla	x				x	x		o
Xema sabini	x			x	o	o
Sterna paradisaea	x			x	o	o
S. hirundo															o	x
Uria aalge	x	o			x	x				o						x	o
U. lomvia	x	o			x	x				o	x					o	x
Lunda cirrhata	?				x	x
Fratercula corniculata		*			x	x
Cerorhinca monocerata	x				x
Cepphus columba											o						x	o
Brachyramphus marmoratus										x					o	x
B. brevirostris										x						o
Synthliboramphus antiquus	x					x				o
Ptychoramphus aleuticus	x			x

Table 16. Number of seabirds of different oceanographic regions having different numbers of categories of food in their diets.

Oceanographic region (domain)	Number of categories in the diets[36]
	1	2	3	4	5-7	7	8+
Bering Sea coastal	3	11	9	6	5	4	5
Bering Sea	2	6	5	7	0	5	0
Alaskan Stream	3	14	14	5	4	4	5
Central Subarctic	1	6	8	4	0	7	0
North American Coastal	3	14	17	6	3	4	6
Total	12	51	53	28	12	24	16
Percent total species (196)	6	26	27	14	6	12	8

It is readily apparent from the foregoing comparisons that much overlap exists in the prey eaten by seabirds within each community. The question whether real competition ever exists is academic. Competition perhaps exists only rarely because seabirds partition resources through use of different feeding methods, selection of different-sized prey, and habitat zonation. Table 18 lists feeding methods (after Ashmole 1971 and Ainley 1977) and the body size and bill length of each species considered in this review. Bill length is usually related directly to body size (Ashmole 1968; Bédard 1969b), but note, for instance, that the longer species of the two kittiwakes has the shorter bill. Body weight would be a better measure of relative size than body size, but few reliable weight data are available for seabirds.

The use of different feeding methods by species in each community grossly assigns birds to feeding at different depths. Thus, whereas shearwaters, puffins, and small gulls (Xema sp., Rissa spp.) overlap almost entirely in prey categories and even prey species, the gulls can capture these organisms only at the surface; the shearwaters capture them at shallow depths; and the puffins capture them at much deeper depths. Direct field observations of this phenomenon are few but Gould (1971) and Sealy (1973a) compared the diets of birds feeding in mixed-species flocks. An example of how even finer divergence in feeding methods helps to partition food resources has been provided by Spring (1971) in his comparison of the two murres. Both species feed by diving to great depths, but the thick-billed murre is able to hover over the bottom and thereby is better able to capture benthic organisms.

Table 17. Number of species feeding at different trophic levels within seabird communities and habitats of the northeastern North Pacific Ocean and Bering Sea. A single species can be represented in more than one level. (Trophic level I = vegetarian, II = secondary carnivore, III = tertiary carnivore, IV = upper level carnivore, Sc = scavenger [II-IV].)

Domain	Oceanic/offshore neritic				Inshore neritic
	II	III	IV	Sc	I	II	III	IV	Sc
Bering Sea Coastal	11	17	1?	10	6	23	18	6	6
Bering Sea	22	21	3?	11	—	—	—	—	—
Alaska Stream	21	19	1?	12	5	28	21	6	6
Central Subarctic	23	22	3?	12	—	—	—	—	—
North American Coastal	25	24	3?	11	3	28	35	7	10
Total	102	103	11?	56	14	79	74	19	22
Proportion	0.38	0.39	0.02[37]	0.21	0.07	0.38	0.28	0.09	0.10

Species	Body length[38] (cm)	Bill length[39] (mm)	Feeding[40] method
Gavia adamsii	63.5	90-91	D
G. immer	61.0	80-82	D
G. arctica	45.7	51-52	D
G. stellata	43.5	51-52	D
Podiceps grisegena	33.0	48-50	D
P. nigricollis	22.9	24-26	D
P. auritus	24.1	23-24	D
Aechmophorus occidentalis	45.7	65-76	D
Diomedea nigripes	71.1	141-144	SS
D. immutabilis	71.1	102-112	SS
Fulmarus glacialis	45.7	36-37	SS
Puffinus carneipes	45.7	41-46	PP
P. creatopus	45.7	41-46	PP
P. bulleri	38.1	38-45	PP
P. griseus	40.3	41-42	PP
P. tenuirostris	38.1	31-32	PP
Oceanodroma furcata	19.0	15	Di,SS
O. leucorhoa	19.0	16	Di,SS
Pterodroma inexpectata	29.2	26-27	SS
Phalacrocorax auritus	68.6	55-57	D
P. penicillatus	73.7	66-71	D
P. urile	71.1	54-55	D
P. pelagicus	55.9	47-50	D
Pelecanus occidentalis	104.0	294-319	P
Branta spp. (bernicla)	43.5	33-36	T
Philacte canagica	45.7	37-42	T
Anas spp.	40.0	32-35	T
Clangula hyemalis	38.1	25-27	D
Histrionicus histrionicus	30.5	25-28	D
Polysticta stelleri	30.5	37-43	D
Somateria mollisima	43.5	45-55	D
S. spectabilis	40.3	31-33	D
S. fischeri	38.1	22-26	D
Melanitta deglandi	35.6	41-44	D
M. perspicillata	40.3	ca. 40	D
M. nigra	35.6	42-47	D
Mergus serrator	40.3	45-54	D
Haliaeetus leucocephalus	80.0	52-54	X
Falco peregrinus	37.5	21-25	X
Phalaropus fulicarius	16.5	22	SS
Lobipes lobatus	15.2	22	SS
Stercorarius pomarinus	43.5	40	SS,A
S. parasiticus	40.3	32	SS,A
S. longicaudus	38.1	29	SS,A
Larus hyperboreus	61.0	55-60	SS
L. glaucescens	55.9	54-58	SS
L. occidentalis	53.0	54-57	SS,Di
L. argentatus	50.8	48-54	SS,Di
L. californicus	43.5	45-50	SS,Di
L. heermanni	38.1	42-46	SS,Di
L. canus	35.6	34-36	SS,Di
L. philadelphia	27.9	30-31	Di
Rissa tridactyla	34.2	39-40	Di
R. brevirostris	38.1	29-30	Di
Xema sabini	27.9	26-27	Di
Sterna paradisaea	38.1	31-33	Di,SP
S. hirundo/forsteri	35.6	36-39	Di,SP
S. aleutica	33.0	33	Di,SP
Uria aalge	35.6	43-47	D
U. lomvia	35.6	39-42	D
Lunda cirrhata	31.8	57-60	D
Fratercula corniculata	29.2	49-51	D
Cerorhinca monocerata	29.2	34-35	D
Cepphus columba	26.7	32-33	D
Brachyramphus marmoratus	20.3	15	D
B. brevirostris	19.0	10	D
Synthliboramphus antiquus	20.3	13	D
Ptychoramphus aleuticus	17.8	19	D
Aethia pygmaea	16.5	8-9	D
A. pusilla	13.3	8	D
A. cristatella	17.8	11	D
Cyclorrhynchus psittaculus	18.4	15	D

The scavengers (generalists) offer a good example of how a range of bird and bill sizes is usually represented among species having similar diets and feeding methods. The progression of oceanic scavenger sizes is graded rather evenly from the black-footed albatross down to the northern fulmar, to the scaled petrel, to the storm-petrel. All these species capture prey that occur only at or near the water surface. Recently Sanger (1973) reported appreciable numbers of glaucous-winged gulls (Larus glaucescens) and herring gulls (L. argentatus), noted neritic scavengers, out in the oceanic realm of the petrel. He presented limited data that suggested an overlap between the diet of these gulls and that of black-footed albatrosses, as noted by Miller (1940). It would not be surprising if these gulls were as much generalists in the oceanic habitat as they are in the neritic. Interestingly, their bill and body sizes fall between those of the albatross and the fulmar, thus in theory enabling them to invade the oceanic habitat without great competition. It is likely that their invasion occurred during historical times and is related to their habit of following fishing boats from shore out to sea (Sanger 1973). If so, the gulls might be assuming from other species part of a previously uncontested resource.

Another interesting group of species that shows close similarities in diet consists of the piscivorous loons, grebes, and mergansers. All these birds, including seven or eight species, apparently feed on fish occurring on or near the bottom in the inshore neritic habitat. Again, however, an even progression in size exists: yellow-billed loon (Gavia adamsii), common loon (G. immer), arctic loon (G. arctica), red-throated loon (G. stellata), western grebe (Aechmophorus occidentalis), red-necked grebe (Podiceps grisegena), and common merganser (Mergus merganser). Most likely then, they select different-sized fish. Another example of this phenomenon is provided by the eight neritic gulls, which are largely scavengers and show a remarkably even progression in bill and body size. Finally, as shown clearly by Bédard (1969a, 1969b) and Harris (1970), alcids of different sizes select different-sized prey, often of the same species.

A final important way in which seabirds partition available resources is by inhabiting different zones. Zonation is especially evident during the breeding season when species common to the same breeding site sort themselves out according to the distances they range for food. This phenomenon was discussed by Murphy (1936), Shuntov (1974), Sealy (1972), Cody (1973), and Scott (1973).

Trophic Relations and Seabird Conservation

The species that appear to have specialized food habits (if further research confirms that indeed they do) are probably very sensitive to vagaries in food availability or are, at least, much more sensitive than other species. Some specialists which also have very restricted distributions would, therefore, be susceptible to localized catastrophes occurring where specialists are concentrated around the food resource. This is proved in the case of the scoters, which are both specialized and rather restricted to nearshore beds of molluscs and have fallen victim to local oil slicks (Smail et al. 1972). An example of another potentially critical situation is that of the black brant, which at certain times of the year concentrate their entire population around eelgrass beds in Bristol Bay, Alaska, where much offshore oil drilling may soon occur.

Birds adapted to feed by diving, with the exception of cormorants, spend most of their time in the water. These species are therefore most susceptible to oiling (Smail et al. 1972), but pursuit plungers (the shearwaters) are also highly susceptible (Point Reyes Bird Observatory, unpublished data). A characteristic of polar and subpolar seabird communities is the high percentage of birds that feed by diving and pursuit plunging. These birds are mostly absent from tropical and subtropical communities because feeding by these methods is not adaptive there (Ainley 1977). Hence, oil pollution has all the potential of rendering maladaptive the principal feeding methods of many polar seabirds.

Another way in which seabird feeding relates to conservation problems concerns competition between birds and man for commercially valuable fishes. A related problem is the mass mortality of seabirds due to man's fishing gear. An acute situation is the drowning of seabirds caught in salmon gill nets (Bartonek et al. 1974; Pacific Seabird Group 1975; Ripley 1975; King et al., this volume). Immediate action is definitely required.

Further, competition between birds and man for the same resource has the potential for disastrous effects on bird populations if humans out-compete the birds and overfish the resource. A classic example, reviewed by Idyll (1973), is the possible collapse of the Peruvian anchovy (Engraulis ringens) fishery; if overfishing and an El Niño should coincide, the Peruvian seabird populations could collapse as well. The California fisheries and apparently the double-crested cormorants that nest on the Farallon Islands have both suffered from the demise of the Pacific sardine (Sardinops caerulea) in the California current (Ainley and Lewis 1974). In regulating fish harvests, fishery organizations should include in their calculations the harvest by creatures other than man (Schaefer 1970), rather than evading the issue by referring to a vague "natural mortality."

---

Finally, fishing by humans can benefit seabirds by removing fish (or whales) that compete with birds for food (Laws 1977). A potential example is that of northern California, where salmon and seabirds both feed heavily on juvenile rockfishes (Fitch and Lavenberg 1971; Point Reyes Bird Observatory, unpublished data). Harvest of salmon should theoretically leave more rockfish available for birds to eat. This sort of situation has not yet been fully documented and definitely warrants further study, especially in such areas as the Bering Sea, where some fish stocks have become depressed due to overfishing (Gulland 1970).

Recommendations for Further Research

Many people realize intuitively that seabirds are important members of marine ecosystems. Although the supporting evidence is not now available, it will be needed if seabirds are to be protected. Emotion alone will not justify the protection of seabirds in an age when the human race moves steadily toward global famine. The job at hand is, in part, to sell seabirds, not just to the public, government officials, executives of oil companies, or fish-packing concerns, but also to marine biologists and oceanographers, for the scientists have the best means to study organisms at sea. We must move away from the concept that seabirds are merely yo-yos of various sizes, shapes, and colors on strings of various lengths that venture forth to sea from the land, grab a quick lunch, and then return to the safety of terra firma. Seabirds are marine organisms and deserve at least as much research attention as that currently given marine mammals.

The information now available on seabird diets is largely presented in terms of the number and volume of various prey species taken. Whereas these data provide the relative importance of prey, fishery data on prey stocks are usually measured in terms of biomass. Thus, it is difficult to relate seabird data to the immense wealth of information on biological oceanography. If we are to recognize the importance of seabirds in the nutrient and energy cycling of marine ecosystems, rather than considering them merely as "yo-yo predators," we must relate them to the total marine community.

The goal of marine ornithologists should be to refine and broaden considerably in detail such studies as those by Sanger (1972), Shuntov (1974), and Laws (1977), who attempted to assess the relations between seabird populations and stocks of other marine organisms for the northern North Pacific, the world oceans, and the Antarctic, respectively. The trophic roles played by seabirds must be studied in detail at the community level year-round before those analyses can be properly refined. Another exemplary work is that done by Brownell (1974), who studied trophic relations of higher vertebrates off Uruguay, including dolphins, pinnipeds, seabirds, and some large fish. In a review study, Sanger (1974) considered the food-chain relations of similar vertebrates in the Bering Sea. These sorts of studies will serve to bring the role of seabirds into perspective with other upper trophic level feeders.

Acknowledgments

We much appreciate the opportunity to participate in the symposium at which this paper was presented. The encouragement and help given by J. C. Bartonek was indispensable. D. G. Ainley's participation in the symposium was supported by the Point Reyes Bird Observatory. This is contribution No. 124 of the Point Reyes Bird Observatory.

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