Chapter IX: Part V: , Birds. U.S. Army, Signal Corps, Washington, D.C. Pages (3)

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Number of categories in the diets[36]
------------------------------------
Oceanographic region (domain) 1 2 3 4 5-7 7 8+
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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
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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 1969_b_), 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 (1973_a_) 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].)

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Oceanic/offshore neritic Inshore neritic
------------------------- ----------------------------
Domain II III IV Sc I II III IV Sc
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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
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Table 18. _Size relationships and feeding methods of major
species in the eastern North Pacific and Bering Sea._ (D = dive,
SS = surface seize, PP = pursuit plunge, Di = dip, P = plunge,
T = tip, x = eats seabirds, A = piracy, SP = shallow plunge.)

--------------------------------------------------------------------
Body length[a] Bill length[b] Feeding[c]
Species (cm) (mm) 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 (1969_a_, 1969_b_) 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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FOOTNOTES:

[15] Present address: U.S. Fish and Wildlife Service, Office of Biological Services, 1011 East Tudor Road, Anchorage, Alaska 99503.

[16] Other incidental prey were squid and atherinid fishes, both at the Farallon Islands.

[17] Other incidental prey were squid and such fishes as atherinids, _Zaniolepis_, _Genyonemus_ and _Peprilus_ at the Farallones, and atherinids, _Trachurus_ and _Heterostichus_ at San Diego.

[18] Other incidental prey were polychaetes at Netarts and the Farallon Islands.

[19] Principal sources: Bent 1925; Cleaver and Franett 1945; Cottam 1939; Cottam and Knappen 1939; Kortright 1942; Mabbot 1920; McGilvrey 1967; Munro and Clemens 1939; Roberts and Huntington 1959.

[20] Other incidental items were the fish _Cololabis_ and _Peprilus_ at the Farallon Islands.

[21] Other incidental items were myctophid fish in northeastern Canada.

[22] Other incidental items included the lamprey _(Lampetra)_ at the Farallon Islands.

[23] Bent (1946) listed "fish" as prey.

[24] Grinnell (1897) listed "fish" as the major dietary component.

[25] Bédard (1969_a_) also listed "fish" as an incidental item.

[26] Other incidental prey were copepods and isopods.

[27] Other incidental prey were pholids in Denmark.

[28] Other incidental prey were copepods and cephalopods in North Atlantic areas.

[29] Other incidental prey were isopods in western North America and fish eggs near Vancouver Island.

[30] Other incidental prey were fish eggs in Denmark.

[31] Offal from wounded whales and seals, and bits of food, primarily crustaceans and fish, from feeding whales are important scavenger foods (Bent 1922).

[32] Other incidental prey were isopods in the North Pacific.

[33] Other incidental prey were the fish _Merluccius_ at the Farallon Islands.

[34] Other incidental prey were isopods near the Pribilofs and in the Chukchi Sea, and amphipods in the latter area; Bent (1921) considered "crustaceans" to be major prey.

[35] Study conducted during period of breeding failure.

[36] These are the "food categories" of Tables 11-15. Items included in diets are not included here.

[37] Proportion based on the arbitrary assumption that half (5) of the 11 species in question catch and eat birds at sea.

[38] Information on body sizes (length) is from Robbins et al. (1966).

[39] Information on bill lengths is from Palmer (1962), Dement'ev et al. (1968), and Friedmann (1950).

[40] Feeding methods are from Ashmole (1971) as adapted by Ainley (unpubl. manuscr.).

Population Dynamics in Northern Marine Birds

by

William H. Drury

College of the Atlantic
Bar Harbor, Maine 04609

Abstract

It seems only reasonable to assume that
populations of marine birds fluctuate even
when not disturbed by man; such fluctuations
would result both from the secondary effects
of species adaptive tactics and from changes
in the marine environment. I briefly review
some human activities and some other natural
processes that have resulted in changes in
numbers and distribution of seabirds and
present a short discussion of theoretical
models which emphasizes that conclusions drawn
or predictions made from models of the dynamics
of populations depend upon the assumptions
about stability that were used in preparing
the models. I then review those special
characteristics of seabirds which are directly
relevant to planning programs intended to
protect seabirds or encourage their increase
and identify several goals for improving our
understanding of the population dynamics and
biology of marine birds. My general conclusion
is that enough is already known to undertake
effective conservation programs, and that time
is pressing.

Seabirds have been categorized as renewable resources in only a few places, although their symbolic value has been recognized for centuries (for example, the medieval poem "The Seafarer" and the designs on Saint Cuthbert's tunic). With the exception of the Russians (Belopol'skii 1961; Uspenski 1956), the Australians (Serventy 1967), and the Icelanders, industrialized peoples have not considered seabirds to be salable and therefore worth managing. Yet during many centuries the seabirds of the northern seas were a major food for coastal and island villages (Bent 1919, 1921, 1922; Fisher and Lockley 1954).

Some biological principles that affect the dynamics of seabird populations are identified in this paper. I believe these principles must form the basis of plans to maintain and increase seabird numbers.

I describe some observations of population changes, review briefly the conflicting theoretical frameworks for population dynamics, and identify some of the biological characteristics of marine birds that affect the way in which population changes occur. The terms "seabirds" and "marine birds" are used interchangeably for those bird species which depend upon salt water for some part of their annual cycle (c.f., the Pacific Seabird Group).

Population Fluctuations

Broadly stated, the populations of northern seabirds have shown marked short-and long-term fluctuations. Most authors have assumed that all such fluctuations reflect human disturbance of the natural system, because of the obvious effects of human predation during the last 200 years.

_Human Impact_

In the centuries before people traveled extensively between islands, seabirds were taken in ways that we judge must have allowed the survival of the colonies (e.g., those at the Faroes or Saint Kilda, those in Iceland and Greenland, or those in the Aleutian Islands and the Bering Strait). We presume either that the populations of island peoples were regulated by shortage of resources other than seabirds or that those who overcropped and eliminated the seabirds suffered the consequences.

Negative Effects

When a sea-going, commodity-oriented way of life evolved, seabirds were killed in huge numbers for such uses as the plumage trade, fish bait, or rendering into oil (Tuck 1960; Fisher and Lockley 1954). Even the elimination of several colonies--e.g., Funk Island, Newfoundland (Tuck 1960); Seal Island, Eastern Egg Rock, Maine (Norton 1921); Muskeget, Massachusetts (Forbush 1929)--may have had little effect on the rate of cropping because those who killed off one source could probably seek out another. As the colonial seabirds became scarce they became more valuable, which stimulated more intensive pursuit of the remnants (Dutcher 1901, 1904).

In some places where seabird colonies did not supply a croppable economic resource, the islands were used for alternative crops with at least temporary commodity value (e.g., foxes were introduced in the Aleutian Islands; Bent 1919). Large herbivores were introduced to supply meat for island residents (e.g., Saint Matthew Island; Klein 1959), as well as pigs, cattle, sheep, goats, and rabbits on islands in the North Atlantic and southern oceans (many authors). Increases in many seabird populations over the last 75 years have been generally associated with relief from predation by humans such as the fowlers, eggers, and plume hunters of the 19th century. Such relief may have been partly responsible for the increase of North Atlantic gannets, _Sula bassana_, and common murres or guillemots, _Uria aalge_ (Fisher and Vevers 1943, 1944; Cramp et al. 1974). On a smaller scale, several population increases along the coast of New England have been recorded following the enactment of protective legislation (Dutcher 1901, 1904; Norton 1921, 1924; Palmer 1949; Drury 1973).

Coulson (1974) argued that in addition to relief from predation, the explosion of the population of kittiwakes _(Rissa tridactyla)_ in this century resulted from access to previously un-occupiable breeding sites. Nesting cliffs and buildings suitable for kittiwake nesting are abundant and now protected from egging or fowling.

Positive Effects

There can be little doubt that human activities have also had marked positive effects in some cases. For example, Fisher (1952) suggested that the North Atlantic fulmar _(Fulmarus glacialis)_ was provided food first by whaling, then by commercial fishing, and that this food allowed the species to increase steadily over the last 3 centuries.

The worldwide increase of gulls (_Larus argentatus_, _L. fuscus_, _L. dominicanus_, _L. ridibundus_, _L. novae-hollandii_) has been credited to availability of food from wasteful human garbage disposal (Murray and Carrick 1964; Fordham 1968, 1970; Harris 1964; Harris and Plumb 1965; Kadlec and Drury 1968; Brown 1967; Mills 1973; Vermeer 1963).

It is hard to dismiss the evidence pointing to the impact of human activities on seabird populations during the last 3 centuries. Yet it would be misleading to assume that without man's interference seabird populations would have remained stable. Success in designing programs of protection and population enhancement must allow for the realities--that seabird populations fluctuate inherently, and that secular changes occur regularly in their environment.

_Impact of Natural Events_

Some population changes appear to result from sudden impacts; other changes are gradual.

Sudden Disasters

Gromme (1927) reported windrows of dead murres in the Unimak Pass and Alaska Peninsula; die-offs of murres in winter storms in the Atlantic and Arctic Oceans were reported by Tuck (1960) and Dement'ev et al. (1968).

Recently some mass mortalities have been associated with specific causes. Bailey and Davenport (1972) reported that starvation caused the die-off of common murres in the southern Bering Sea--Bristol Bay area. Foul weather, which apparently inhibited feeding between 19 and 23 April 1970, culminated in an intense storm. Similarly in late winter 1969 bad weather in the Irish Sea, combined with strains of molt and perhaps contamination with industrial chemicals, seems to have contributed to mass mortality of the same species (called common guillemot in Britain; Holdgate 1971). The seabird victims of this event had metabolized their body fat and as a result, polychlorinated biphenyls (PCB) and other industrial chemicals passed into livers, kidneys, and brains. Again, a storm at the end of a period of stress seems to have been more than the birds could tolerate.

A further example of a die-off of waterfowl apparently brought on by starvation was given by Barry (1968), who estimated that about 100,000 king eiders _(Somateria spectabilis)_ died when they arrived before the ice broke up in the Beaufort Sea in spring 1964.

Diseases have produced massive die-offs in marine birds. Fowl cholera caused high mortality in nesting common eiders _(Somateria mollissima)_ in the Gulf of St. Lawrence in Quebec (Reed and Cousineau 1967) and in Penobscot Bay, Maine, in the early 1960's (H. Mendall, personal communication). Poisoning from a "red tide" (a bloom of the dinoflagellate _Gonyaulax tamerensis_) caused a die-off of black ducks _(Anas rubripes)_ and herring gulls on the coast of New England in 1972. Similarly a die-off of shags _(Phalacrocorax aristotelis)_ on the east coast of England was caused by a "red tide" (Coulson et al. 1968). During a period of 1 week 90% of the shag nests on the Farne Islands in Northumberland were deserted and about 80% of the breeding population died.

Gradual Declines

When the new volcanic island of Bogoslov emerged in the western Aleutians, Preble and McAtee (1923) reported that it was colonized by large numbers of pigeon guillemots _(Cepphus columba)_, but in the following decades the guillemots have steadily decreased (G. J. Divoky, personal communication). As a further example, the nesting population of Atlantic puffins _(Fratercula arctica)_ in the Atlantic has declined over the past several years, especially those nesting on the Outer Hebrides (Flegg 1972; Harris 1976).

It is difficult to find seabird species whose nesting grounds have not been affected by humans but whose numbers have been censused. The best illustrations of secular changes in relatively constant habitats are probably those available in the British Trust for Ornithology's breeding censuses of songbirds. Songbirds are short-lived and their populations change on relatively short time scales. The northwestern European landscape has remained relatively constant for the last 75 years, yet there are observable decade-long trends--for example, of willow warblers _(Phylloscopus trochilus)_ and dunnock _(Prunella modularis)_. There are detailed data on population changes in great tits _(Parus major)_ through the work of Kluyver (1951), Lack (1964), and Perrins (1965).

Effects Reflecting Environmental Change

Nelson (1966) argued that the increase of gannets in the North Atlantic during this century has been related to increasing temperatures rather than (as usually ascribed) to increased food from fish damaged or escaped during commercial fishing.

Ainley and Lewis (1974) described a particularly interesting example of the effects of environmental change on seabird populations. The events begin with the decrease of seabirds on the Farallon Islands off California as a result of human depredations. Even after fowling was made illegal, the populations of murres, double-crested cormorants _(Phalacrocorax auritus)_, and especially of tufted puffins _(Lunda cirrhata)_ and pigeon guillemots continued to decline as a result of oil pollution. During the last 3 decades the smaller species of seabirds nesting on the Farallons, such as rhinoceros auklets _(Cerorhinca monocerata)_, have increased rapidly and the authors suggest that their increase was abetted by an increase in the small prey fish, northern anchovy _(Engraulis mordax)_. One of course expects predators to be affected by changes in the abundance of their prey. During this same period, larger species of seabirds such as double-crested cormorants and tufted puffins have failed to recover their numbers, and the authors speculate that this failure is related to a decrease of the larger prey fish, Pacific sardine _(Sardinops caerulea)_.

A widely publicized impact of environmental fluctuation upon seabird populations is that of the northeast wind, El Niño, off the Peruvian coast. This wind pushes the upwelling Humboldt Current water offshore and causes mass mortality in the Peruvian anchovies _(Engraulis ringens)_ and, as a consequence, a die-off among the millions of seabirds such as Peruvian guanay cormorants _(Phalacrocorax bougainvillii)_ and Peruvian boobies or piquero _(Sula variegata)_ which feed upon them (Murphy 1936).

Theoretical Considerations

Can useful generalizations be drawn from these observations on population changes? Can a model be constructed of the forces which drive population changes or of population-habitat interactions which keep populations from extinction? Some conflicting theories and assumptions of population dynamics are examined and discussed below.

_The Assumption of Population Stability and of Closely Attuned Density-dependent Mortality_

During the 5 decades before 1970, it was widely accepted that most animal populations were generally stable and saturated before the arrival of the white man. Although a few field biologists vigorously dissented, "establishment" ecologists regarded fluctuations as a departure from the norm, and as such, a hazard to the population. Many theorists of both evolution and ecology argued that adaptations were required to damp fluctuations or the fluctuations would become "random walks" and the population would rapidly become extinct. As a consequence, relatively all theoretical models included stability as a central assumption.

• The basic element of this theoretical complex has been the Lotka-Volterra formula for a logistic curve of population growth and stabilization. According to this formula it has been reasoned that by establishing the inherent rate of increase of a population (i.e., its average natality relative to mortality, or _r_) and by measuring the carrying capacity of the environment (which is the density of the population at saturation, or _K_), one can predict the maximally productive population size, and maximum rate of production of new individuals (or maximum sustained yield). These assumptions have supplied the theoretical framework for virtually all game management and many fisheries practices.

Once stability was assumed, a mechanism for maintaining stability was necessary. This mechanism was found in an interaction between the population and the environment, called density-dependent mortality (Nicholson 1933). The impact of this feedback has been assumed to cause the point of inflection of the "sigmoid curve" and to regulate the density "at equilibrium."

Populations growing in relatively isolated or closed systems have been observed to follow a sigmoid curve toward a steady state. We have data on the growth of several Massachusetts gull colonies which show this type of short-period rapid increase followed by a long sequence of shallow oscillations (Drury and Nisbet 1972). But usually observations have been terminated at about the time the population passed through the point of inflection.

• Lack (1954) accepted the principles formulated by Lotka-Volterra and hence viewed Nicholson's (1933) density-dependence as logically necessary. Lack (1948, 1954) argued that reproductive effort (clutch size or litter size times the number of broods) must be as large as the parents can successfully raise to independence because these biological characteristics are directly subject to natural selection. He argued that because reproductive potential is excessive (Darwin 1859), mortality must be density-dependent if a population is to avoid fluctuations. The only adequately density-dependent regulating process he accepted was the population's response to its food supply (Lack 1954). In fact, for many years Lack rejected Kluyver and Tinbergen's (1953) hypothesis that territory could act as a control on population size in birds because, he argued, territories were compressible and therefore allowed wide fluctuations. To his credit, however, Lack eventually acknowledged this mistake.

The first defect in the concept of "carrying capacity" is the idea that populations have "mechanisms" or "institutions" (Wynne-Edwards 1959) by which the population is kept stable at the carrying capacity in a stable habitat.

The second defect in the concept of carrying capacity is that it presupposes a stable environment. During the early decades of the 20th century most climatologists believed that a departure from the norms of a regional climate set processes in motion which would return the climate to normal. During the last decades, however, climatologists and oceanographers have shown clearly that environments are continuously in flux.

_An Attack on Density-dependent Mortality_

Some theorists rejected the concept of carrying capacity as soon as it was formulated. Andrewartha and Birch (1954) predicted fluctuations would be undamped by inherent population mechanisms but rather would be controlled by external forces indifferent to the density. Their supporting data were drawn from field studies of insects in arid climates. Some of their ideas are directly relevant to seabirds; for example, their assertion that in many cases limits to carrying capacity of the habitat are not set in a way responsive to the density of the population. The number of occupiable ledges on a seabird cliff are fixed and when they are full no more birds can breed there regardless of the amount of food available. For another example, some biological processes act in a way that reinforces fluctuations. Predation can act in this way in the relatively closed system of a seabird colony; i.e., the smaller the prey population the larger the percentage taken by the predators. The importance of predation as a selecting factor is shown by the adaptations marine birds and waterfowl make to avoid it. The fact that large colonies of seafowl are usually concentrated on isolated, predator-free islands is one obvious case (Lack 1966).

Although their ideas are useful in understanding changes in many species, primarily insect populations, the generality of Andrewartha and Birch's (1954) hypothesis is weakened because it conflicts with detailed studies of seabirds which show that in many cases local food resources do limit breeding success. Ashmole (1963) showed this for tropical terns, and Hunt (1972) for some colonies of herring gulls on the New England coast. Nettleship (1972), studying the effects of herring gulls on Atlantic puffins, showed that the effect of harassment and stealing food from the parents was to reduce the amount of food brought to the young and thus reproductive success. In those parts of the colony where gulls were numerous or where the puffins were at a disadvantage in escaping from gulls (i.e., flat rather than steep slopes) the reproductive success of puffins was significantly lower than in areas away from the gulls.

The literal application of Andrewartha and Birch's general ideas also conflicts with observations on subtle adaptations some waterfowl have made to counter predation.

Barry (1967) described the density-avoiding adaptations of arctic-nesting geese to evade predation--specifically by foxes. Black brant _(Branta nigricans)_ nest on low coastal or delta islands seeking to escape by remoteness. Snow geese _(Chen caerulescens)_ are colonial on large, flat areas, seeking protection in numbers. White-fronted geese _(Anser albifrons)_ are solitary nesters on inland swamps, seeking to be "over-dispersed" among scrub willow.

Common eiders, black scoters _(Melanitta nigra)_, tufted ducks _(Aythya fuligula)_, and other ducks select gull colonies as nesting habitat. Although there is little doubt that the ducks choose gull colonies for nesting, there is some doubt as to the reasons. Finnish biologists (summarized by Bergman 1957; Hildén 1965) have concluded generally that gulls protect the duck nests from predation by hooded crows _(Corvus corone)_.

_The Assumption that Fluctuations Are Generally Present_

Recently theorists have built models based on assumptions that fluctuations are a general characteristic of population dynamics, such as Gilpin's (1975) model describing multi-phased oscillations. He took account of the fact that fluctuations (and models) become more complex as more species and nonlinear effects are included. May and Leonard (1975) emphasized that the effect of nonlinearities is to make it impossible to speak even in principle of the equilibrium point of a community. They pointed out that even though the model is deterministic (i.e., assumes that the system will come to equilibrium) the oscillations are so complex that they may appear to be random, and it may be a very long time before the system returns to a position near its starting point. "On the other hand a truly random ecological system could always be fitted by a suitably ingenious limit cycle. This suggests that ecological analysis which does not consider component processes must be viewed with great suspicion" (Gilpin 1975). May and Leonard (1975) and Gilpin are both making a familiar point--that neither the logic nor the interactions described in a formula will describe biological reality unless the assumptions are correct. They are also making a different point--that an ingenious mathematician can create a formula to describe almost any operation (whether its workings are systematic or random), and the formula may seem to work.

Gilpin's moral is that one cannot learn very much that is helpful by studying fluctuations as such. One must study the factors controlling populations. This is a very old idea.

It would appear that defining carrying capacity and inherent rate of increase will not be very instructive in managing seabird populations other than in speculating upon what might be ideal upper limits. It can also encourage the musty sophistry that when a population increases beyond this abstract carrying capacity it "needs" to be hunted to prevent overcropping resources and damage to itself through a population decline. But we will not have the time to carry out detailed studies of life histories seeking for critical population-habitat interactions over several fluctuations for each species involved in a disaster before designing programs to help seabird populations to build up their numbers.

General Characteristics of Marine Birds and Waterfowl

Because general theory does not seem to work and because detailed studies take too much time, I conclude that it is necessary to identify certain general principles upon which to base applied programs. These categories of knowledge include: (1) how vulnerable certain categories of seabirds, waterfowl, and shorebirds are to specific types of disasters, (2) how quickly their numbers build up after they have been reduced, and (3) at what stages we can help them best (i.e., at the breeding grounds, at the winter gathering grounds, or on migration). I believe that we already know enough to design effective programs and to begin work. To this end some characteristics of seabirds are identified which determine the population structures and ways in which their numbers respond to changes in the environment.

_Habitat_

Although the shallow oceans, islands, and seashores are among the most permanent features of the earth in general, the details of their numbers and distribution change rapidly. Sandy shores are obviously being reworked even in the short span of a single lifetime. Distribution of islands and the sediment load, extent, and strengths of currents vary constantly in space and change with time.

The food that seabirds use is patchy and subject to both short-and long-term fluctuations in numbers and shifts in geography. Suitable breeding habitat is scattered, and in many places where oceanic conditions provide a good food supply there are no nesting sites. Consequently, seabirds aggregate in colonies, often dense, and the colonies are clumped for geographical as well as biological reasons.

Lack (1966) discussed some general features of how the breeding adaptations of seabirds are adjusted to the distances the birds must go to find food. The species which feed close to the nest characteristically establish isolated territories or nest in small groups, and they accept many different kinds of nesting substrate. Their clutch sizes are large, individuals move nesting sites readily, and their young grow rapidly compared to the species which feed far at sea. Species which feed far at sea aggregate in large colonies. These species are often rigid in their requirements for suitable nesting sites, their clutches are usually limited to one egg per season, their young grow slowly, and there seems to be strong attachment to traditional colony sites.

_Breeding_

Ashmole (1963) suggested that the clutch size of some oceanic birds is small and colonies occupy only part of the available habitat because food resources within efficient commuting distance of the breeding site are limited. We can see this effect in the usual failure of common terns to raise a third chick, even in the colonies that are surrounded by favorable habitat (Nisbet 1973). Herring gulls whose colonies are close to sources of human refuse raise more young than do those whose colonies are at some distance (Drury 1963; Kadlec and Drury 1968; Hunt 1972).