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

by Unknown

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).