Chapter X: Part V: , Birds. U.S. Army, Signal Corps, Washington, D.C. Pages (4)

Ashmole (1963) suggested that during the course of the breeding season the birds exhaust the available food supply. The validity of this suggestion is reflected in the long distances some species (petrels, boobies, murres, dovekies) go for food to feed their young. One would therefore expect that early nesting pairs would be more successful, and this seems to be the case in herring gulls (Nisbet and Drury 1972), kittiwakes (Coulson 1966), and red-billed gulls, _Larus novaehollandiae_ (Mills 1973).

If food is in short supply and parents have to seek over a wide area for food so that they can bring back only a little food at long time intervals, one would expect these birds to have a small clutch and their young to grow slowly, as is the case. One would also expect seabird colonies situated near oceanic currents to be larger and more successful because food is continuously renewed. Conversely, one would expect colonies next to still waters to be smaller and less successful.

The small clutch size of seabirds means that when a population has been reduced, it will grow slowly toward its former abundance. The growth rates of seabird populations on the New England coast since their release from human predation reflects this. Species such as black guillemots with only two eggs per clutch and herring gulls with three eggs per clutch have increased more slowly than have the populations of common eiders or double-crested cormorants both with three to six eggs per clutch (Drury 1973).

If the species that nest in colonies show a high degree of site tenacity, they are not likely to reestablish a colony after it has been eliminated. An exception to this is the food subsidy provided by man, which seems to have been important in creating a nonbreeding population of herring gulls large enough to form a "critical mass" for the formation of a new gullery.

_Age Structure_

Because the main element of population size--the number of breeding adults--is limited by the number of breeding colonies and the food available to those colonies, one assumes that the total numbers of seabirds is much less than could be supported by the larger areas of productive oceans. Hence one suspects that there is lessened competition for food outside the breeding season and that lack of competition for food is a major reason for seabirds being long-lived, often to extremes little suspected until recently. Mortalities of 10-12% per year are common, and some as low as 4% (wandering albatross, _Diomedea exulans_; Tickell 1968) have been recorded.

In contrast, songbirds with large clutches, such as the titmice studied by Kluyver (1951), produce a large number of young with whom they and other adults must compete for food during the winter period of food shortage. Because the titmice are permanent residents, they occupy all of the available habitat throughout the year. Hence titmice suffer intense intraspecific competition, which shortens the survival of adults. Kluyver's experiments (1966) with nest boxes used by a closed population of great tits on Vlieland, The Netherlands, showed that by artificially reducing clutch size the survival of adults was increased.

Similar competition for the few territories available on marshes and consequent shortened life expectancy, can be expected in waterfowl with large broods. The effect should be less marked for geese with smaller clutches that nest in less confined habitats.

The long life span of seabirds means that a population will have a large component of older age categories; this characteristic has several implications:

• It means that the population can survive years of reproductive failure without the observable immediate effects that would be manifest in titmice, grouse, or rabbits. Near failure of reproduction during a breeding season among arctic seabirds at Bear Island was reported by Bertram et al. (1934). Many similar observations have been made since then: Pitelka et al. (1955) reported such a case among skuas and gulls at Point Barrow, Drury (1961) for greater snow geese _(Chen cerulescens atlantica)_ at Bylot Island, Jones (1970) for black brant gathering at Isambek Lagoon on the Alaska Peninsula, and D. A. Snarski (personal communication) for kittiwakes at Cook Inlet. Reproductive failure can sometimes be chronic, as observed by Nisbet (1972) for terns at Cape Cod, Massachusetts, or by Drury (1963) and Hunt (1972) for herring gulls on the outer islands on the coast of Maine.

When reproductive failure becomes chronic as observed on peregrine falcons _(Falco peregrinus)_ by Hickey (1969) and in ospreys _(Pandion haliaetus)_ by Ames and Mersereau (1964), the population of adults may hold on for a number of years without evident decline. Damage to the structure of the whole population may be serious before any numerical results are evident.

• Although there may not be intensive competition for food in the habitat away from breeding colonies, there is intense competition for food and breeding sites at and around the colonies. Hence age and previous experience in seabirds assume importance in establishing territory and in breeding success. Associated with this is the tendency for immature birds to delay breeding until they are several years old and for the immatures to remain on feeding grounds at some distance from the colonies. In some cases young birds may "hang around" breeding colonies and even feed some of the young. When young birds do first breed they usually lay smaller clutches and raise fewer young than do older birds. The importance of age and experience upon breeding success has been well documented for kittiwakes (Coulson 1966) and red-billed gulls (Mills 1973).

The fundamental biological importance of this delayed maturity seems to be emphasized by the persistence for several years of immature plumages, so clearly identifiable that even a human observer can recognize the age of an individual. One assumes such an evident feature must have adaptive significance.

_Wintering Grounds_

When colonial nesting seabirds leave their breeding islands for their wintering grounds, their identification with that island is lost as far as population effects are concerned, because birds from many colonies mingle on the wintering grounds. Major mortality takes place on the wintering grounds and must therefore act on the species population as a whole rather than differentially on individuals associated with especially dense colonies. Such a direct relation between colony density and mortality would be necessary for density-dependent mortality to regulate the number of birds on a breeding colony. Conversely, one cannot expect that all colonies will decrease equally because mortality should be equally distributed if all the population gathers on a common wintering ground. Thus density-dependence acts only in a very general way upon the sum of animals considered as an abstract entity--the population.

In fact, on the wintering grounds, as shown by a graph of numbers of gulls reported on Christmas Counts on Cape Cod, Massachusetts (Kadlec and Drury 1968), herring gulls are very responsive to local conditions and move several tens of miles to gather at favorable feeding sites. An aerial survey of the gulls on the East Coast of the United States (Kadlec and Drury 1968) showed that more than half of the gulls were gathered near major food sources in large metropolitan districts. Most of the remainder were gathered near small fishing ports. Very few were scattered along the shoreline in what one assumes is the traditional gull habitat. Later analyses of the relation between the distribution of banding recoveries of birds in their first winter and the distribution of immatures as found on this winter census (Drury and Nisbet 1972) suggested that proportionately more first-year gulls died in those areas where the birds were sparsely distributed than died in the crowded metropolitan areas.

These results suggest both that there is not a direct feedback between reproductive rate and mortality, and that mortality may even be inversely density-dependent on wintering grounds. This last runs counter to traditional ecological ideas that density causes a change in mortality rate. The idea that individuals gather where "living is easy" and mortalities are low is consistent with the theory of natural selection. One would not expect the food of the gulls to be evenly distributed, and one would expect individuals to move away from areas where food is scarce and mortality is high.

_Differences in Breeding Success Between Colonies_

Breeding success has been shown to vary among individual pairs of gulls (Drost et al. 1961). Certain groups of individuals nesting in patches within a single colony have greater breeding success than do others (Coulson 1968; Drury and Nisbet, in preparation). Differences in breeding success also occur between colonies (Frazer-Darling 1938; Kadlec and Drury 1968; Drury and Nisbet 1972). Some colonies reproduce consistently better than others--for example, the gull colonies close to fishing ports and metropolitan areas. Other colonies produce consistently fewer young, such as the colonies on the outer islands in the Gulf of Maine (Drury 1963; Kadlec and Drury 1968; Hunt 1972). The populations of successful colonies grow while the numbers of unsuccessful colonies decline, even during a period of general population increase (Kadlec and Drury 1968).

The difference between success and failure, growth and decline, appears to lie in the food available. Colonies increase where breeding success is high and decrease where breeding success is low. One important reason seems to be that adult gulls may move to a more productive colony even after they have nested with another colony (Drury and Nisbet 1972; Kadlec 1971). Such adaptations can be viewed as adjustments by which individuals meet the requirements of an environment in which the availability of food and other necessities is patchy and shifting.

_Dispersal_

In general terms, the willingness of some individuals to disperse while the majority of individuals remain loyal to a colony can be considered a major mechanism of population maintenance. If conditions deteriorate seriously at one place so that the local populations decline or disappear, dispersal from other centers can be expected to repopulate the area as soon as local conditions again become suitable. This subject has been treated in more detail by Drury and Nisbet (1972) and Drury (1974_b_).

Occupation of new, or return to former, nesting sites has been recorded in detail for fulmars _(Fulmaris glacialis)_ by Fisher (1952) and for herring gulls by Kadlec and Drury (1968). Dispersal is also known for waterfowl. Hansen and Nelson (1957) reported that of some 8,000 brant banded in midsummer on the Yukon delta 8 were recovered in northern Siberia and 28 in northern Alaska and arctic Canada. They suspected that pairing on the wintering grounds was responsible for the change in breeding areas, a change that would not be expected among other North American species of geese. Similarly, wide dispersal seems to occur in pintails _(Anas acuta)_, mallards _(Anas platyrhynchos)_, and wood ducks _(Aix sponsa)_.

The general tendency for some individuals to disperse and the frequency of "extra limital" breeding attempts is especially well established in the Bering Sea region, in part at least because vagrants from Siberia or North America are readily identified as such. In the Aleutian Islands, Emison et al. (1971) and Byrd et al. (1974) have enumerated the nesting vagrants. For the Pribilof Islands, Kenyon and Phillips (1965), Sladen (1966), and Thompson and DeLong (1969) have recorded the repeated appearance of birds of Siberian distribution, and Fay and Cade (1959) and Sealy et al. (1971) did the same for St. Lawrence Island.

One can conclude that a few individuals are constantly trying to settle in new geographical areas. As climatic and habitat conditions change, some populations are able to become established; for example, southern species such as mockingbirds _(Mimus polyglottus)_, cardinals _(Cardinalis cardinalis)_, and tufted titmice _(Parus bicolor)_ have settled in southeastern New England during the last 2 decades. These southern species have received much publicity. But at the same time, a less publicized dispersal of white-throated sparrows _(Zonotrichia albicollis)_, hermit thrushes _(Catharus guttatus)_, and dark-eyed juncos _(Junco hyemalis)_ has resulted in new nesting records of more northerly species, also in southeastern New England.

The ability (or lack of ability) of some organisms to expand their ranges over time has been a subject of consideration for a number of years by plant and animal geographers. An important botanical paper on this subject in the Bering Sea region was presented by Hultén (1937), who analyzed the ranges of plants of the area of Kamchatka, eastern Siberia, Alaska, and northwest Canada, showing that diverse floras occur in some restricted geographic areas. He called these areas "refugia," and postulated that many species had survived Pleistocene glaciations in them because these refugia remained ice-free. He, like Fernald (1925), was puzzled as to why only certain species had been able to expand their ranges outward from these "areas of persistence," while other apparently more "conservative" species were unable to do so. Similarly, there appear to be conservative endemic bird species of the Bering Sea region: the extinct Commander Islands cormorant _(Phalacrocorax perspicillatus)_, Steller's eider _(Polysticta stelleri)_, spectacled eider _(Lampronetta fisheri)_, emperor goose _(Philacte canagica)_, whiskered auklet _(Aethia pygmaea)_, least auklet _(A. pusilla)_, parakeet auklet _(Cyclorrhynchus psittacula)_, Aleutian tern _(Sterna aleutica)_, red-legged kittiwake _(Rissa brevirostris)_, bristle-thighed curlew _(Numenius tahitiensis)_, long-billed dowitcher _(Limnodromus scolopaceus)_, surfbird _(Aphriza virgata)_, black turnstone _(Arenaria melanocephala)_, rock sandpiper _(Calidris ptilocnemis)_, and western sandpiper _(C. mauri)_.

The ranges of horned puffins _(Fratercula corniculata)_, Kittlitz's murrelet _(Brachyramphus brevirostris)_ and, perhaps, crested auklet _(Aethia cristatella)_ suggest that some species of "Beringian" seabirds have expanded their ranges from Hultén's (1937) "refugia."

_Dispersal and Regional Persistence of Marginal Populations_

The presence of several sub-elements of a species population and, therefore, the opportunity for dispersion among alternative breeding sites may be an important factor in the regional persistence of a species on the margin of its range, as illustrated by the history of laughing gulls _(Larus atricilla)_ in New England.

Between 1875 and 1900 there were fewer than 50 laughing gulls in Massachusetts (Mackay 1893) and about 35 in Maine (Norton 1924). In Massachusetts the laughing gulls all settled on one large island, Muskeget, where by 1940 there were about 20,000 pairs (Noble and Würm 1943). Meanwhile, in Maine the population had been disturbed by sheep and men and had shifted about among seven islands. The Maine population grew to only about 350 pairs by 1940 (Palmer 1949).

The laughing gull population in both States has decreased since 1940. In Massachusetts, where all pairs occupied one island, the population had fallen to about 250 pairs by 1972, but the Maine population, still divided into five colonies, stabilized at 250 pairs (i.e., the same as instead of only 1% of the Massachusetts population).

Use of General Principles in Solving Conservation Problems

Game biologists have successfully maintained the populations of hunted animals by using a number of classical principles of game management. They have controlled mortality by regulating kill and have increased standing stock by improving habitat on a local scale. This seems to have worked in species which are short-lived, have large clutch sizes or litters, and which occupy mosaics of highly productive "successional habitat." Seabirds, however, contrast with these species in a number of important biological characteristics. They have small clutches, postpone breeding until they are several years old, and are subject to periodic or chronic reproductive failures. Therefore, their populations are skewed toward older animals and replacement of lost individuals is slow. Many seabirds, like some geese, have a high level of site tenacity and thus may resist recolonization or fail in the attempt to recolonize a breeding site once eliminated from it. In those species studied it appears that the breeding birds at a small percentage of colonies are responsible for a large proportion of the annual crop of young. It is probably dangerous, therefore, to risk either damage to or elimination of well-established colonies.

Studies of kittiwakes by Coulson and White (1958, 1961), sooty terns _(Sterna fuscata)_ by Ashmole (1963) and Harrington (1974), Atlantic puffins by Nettleship (1972), and Cassin's auklets _(Ptychoramphus aleutica)_ by Manuwal (1974), and the practice of eider "farming" in Iceland indicate that the number of available territories or breeding sites may limit the size of a population and that populations can be increased by increasing the number of sites available. This suggests one way in which direct steps can be taken to encourage the numbers of breeding seabirds. Other studies indicate that seabirds will move into synthetic habitat such as created by the window ledges on buildings (Coulson and White 1958) or the islands created by dumping spoil from channel dredging operations (Buckley and Buckley 1971, 1975; Soots and Parnell 1975).

Most generalizations of population biology have been derived from the study of insects, songbirds, or game species. It seems inadvisable to assume that those principles will apply to seabirds without modification. For example, predation by gulls and ravens may have a disastrous effect on a seabird colony at low colony density but have progressively less impact as the colony size and density increase. Fox predation may have important effects over most ranges of prey density because the presence of foxes has important psychological effects.

The habitats of seabirds include elements in which birds are widely dispersed (feeding areas) and others in which birds are crowded and narrowly localized (nesting sites). Thus effective programs of conservation should include guarantees that a number of colony sites be available in as widely dispersed a pattern as possible. Each productive feeding ground should, if possible, have several colony sites available.

We have argued elsewhere (Drury and Nisbet 1972; Drury 1974_a_) that one of the chief defenses any population has against extinction is the combination of being divided into a number of population centers with having some movement of individuals between the centers, but not too much. Because it is highly improbable that a single catastrophe will affect more than a part of a species' range at any time, the more numerous and widely scattered the partially independent segments of a population are, the better the species is insured against extinction. This, of course, suggests that the size of each colony may be less important for long-term survival than is the total number of colonies.

One intuitively concludes that "conservative" species, such as those endemic to the Bering Sea region (whose dispersal and colonizing mechanisms seem to be poorly developed), are especially vulnerable to the effects of local population crashes. These "local" species therefore deserve special consideration.

I would like to emphasize two points to be included in designing a "management" program:

• It seems that the most promising management techniques will be built upon ensuring the health of colonies and the associated feeding areas at which reproductive success is high enough to "export" young. Thus it is useful to identify those colonies which are exporting young and to give special care to their preservation. As populations of prey species change locally, so will the success of the local nesting birds. A colony which is thriving at one time may be barely maintaining itself at another (Ainley and Lewis 1974), or it may decrease, as in the case of "guano birds" during El Niño years in the Peru Current.

• Because centers of abundance of marine birds shift (Fisher 1952; Drury 1963, 1974_a_), it will be prudent to plan for large areas and over long periods of time. Harrison Lewis, a pioneer in seabird management in eastern Canada, said (personal communication) that just as soon as he got approval of a new seabird sanctuary through the long corridors of the distant government bureaucracies in Ottawa, the birds would move to a new island and he had to start the process all over again.

The objective is to maintain a variety of colony sites for populations to move among as local patterns of productivity in the shallow sea shift.

Goals for Research on Population Dynamics of Seabirds for Purposes of Conservation

1. To learn the distribution and relative importance of seabird colonies, the number of pairs nesting and nonbreeding individuals at each colony, and the timing of breeding activities for each geographical region. The most important step toward conserving marine birds is to get public ownership and protection for their breeding grounds.

2. To understand the life cycle of key species. Three needs are clear:

a. To identify key species whose biological characteristics can conveniently be studied and measured. Studies of these species may be useful in monitoring the "health" of seabird breeding areas.

If it is established that the reproductive success of certain species varies similarly in response to changes in their marine habitat (such as black-legged kittiwakes and horned puffins), one could use key species (black-legged kittiwake) to assess the performance of those species in a colony whose breeding success is difficult to measure (horned puffin).

b. To develop efficient and practical ways of censusing and measuring productivity of crevice-, scree-, and hole-nesting species such as puffins and auklets.

c. To establish annual differences in reproductive success and mortality rates by age classes of the key species, and from these to identify rates of population turnover so as to be able to predict the effects of mass mortalities.

3. To learn enough about the differences in behavior and productivity among colonies to establish which colonies produce surplus young and which have low productivity. At first, maximum efforts for conservation should be concentrated at those sites which produce surplus young.

4. To learn about colonial behavior. Two needs are apparent:

a. To know enough about the lives of individually marked birds of known age so as to be able to infer the behavior of population elements at all stages of their life cycle.

b. To know enough about the lives of subadult birds to understand what proportion of subadults visit and become established at breeding sites, why the subadults visit the breeding sites and what effect their presence has on the territories and breeding success of their neighbors and biological relatives.

5. To know enough about places where seabirds, waterfowl, and shorebirds gather on migration and during the winter to identify those areas which need special protection from effects of economic development.

a. It is important to determine the areas where marine birds gather at sea when they are away from their breeding grounds. What factors of habitat and food supply make certain places preferable to others? What is the relation between gathering grounds and underwater topography (banks and edges of the continental shelf)? What are the seasonal and annual differences in preferred gathering grounds? What special hazards exist, such as unusual extent of sea ice or exceptional storms?

b. It is important to plot coastal areas where waterfowl and shorebirds gather on migration, for molting, and during the winter. Which open leads in the ice and patches of open water at the mouths of rivers are of especial importance in spring? What shorelines and beaches act as "leading lines" during migration? Which capes and points result in concentrated overflights of migrating waterfowl, and hence are locations of unusually high kills by hunters? What wetlands, bogs, coastal ponds, lakes, and lagoons are used as gathering grounds and to what extent do waterfowl and shorebirds exchange between gathering grounds? How much redundancy of wetlands is needed to make the wetlands system maximally productive for waterfowl and shorebirds?

Answers to these questions will identify which geographic areas deserve special protection during development. The answers will also identify the kinds of influences which might lower the contribution of each critical area to the survival of seabirds, waterfowl, and shorebirds. Areas identified as important under these categories must be included in policy decisions related to land-use planning and management.

6. To learn more about the effects of varying quantities of food on breeding behavior and performance:

a. What are the effects of food abundance in early spring on date of laying, clutch size, and egg size?

b. What effects do storms have at different stages of the reproductive schedule?

c. What effects do quantitative and qualitative (species composition of prey) changes in food supply have on the survival of chicks?

d. What are the similarities and differences between what parents eat and what they feed their chicks?

Although this is important basic biological knowledge, it contributes little to conservation efforts because food differences result from changes in the ocean over which humans can have little effect.

7. To learn more about prey species and their availability to marine birds:

a. To know more about the breeding areas, reproductive rates, growth rates, and routes of dispersal of the major prey food species. In most areas a few species of teleost fish (e.g., _Ammodytes_) or Crustacea (e.g., copepods, euphausids, mycids, or amphipods) make up most of the food of marine birds. Yet, the barest minimum is known about the biology of such species. A good first estimate of the "condition" of the marine environment can probably be made by measuring reproductive rates and growth rates of these key prey species. Hence an efficient (though indirect) way to measure those rates may be by monitoring reproduction of birds.

b. To know more about the density and distribution of key prey items season by season, and to learn more about the relation of their abundance and distribution to their availability to birds, as Bédard (1969) showed for _Calanus_ to least auklets, and _Thysanoessa_ to crested auklets.

There are some indications that the population size of prey items can vary widely without having a marked effect on the numbers of their predators. Does commercial fishing for the large, predatory fish have a measurable effect on the food available to marine fish? Do the large pollock and salmon fisheries (high seas) make zooplankton available to smaller alcids? Do marine birds affect a fishery?

c. To know more about the oceanography of continental shelf waters, more specifically the waters between 6 and 60 m deep. The shallow waters of continental shelves are some of the most productive of sea waters, but are among the least studied. Although some species (black-legged kittiwakes, tufted puffins, and fulmars) move into deep waters, many species of marine birds of northern waters gather in large numbers on preferred feeding grounds at or near the edges of continental shelves during their winter season (Fisher 1952; Tuck 1960).

8. To know more about the potential effects of proposed developments on seabirds and waterfowl.

a. To prepare models which will predict probabilities of contamination of breeding and feeding areas (summer, winter, and during migration) using existing knowledge of

(1) areas of proposed mineral development;

(2) areas that will be influenced by secondary development such as dredging new harbors, laying subsurface pipelines;

(3) tidal and oceanic currents;

(4) numbers of marine birds or waterfowl using specific geographic areas and habitats (e.g., waters below nesting cliffs, feeding grounds, wintering grounds, and gatherings during migration);

(5) the distribution and patchiness of habitats (i.e., the redundancy among and within habitats and the degree to which populations exchange between alternative habitats);

(6) the biological importance of species in local ecosystems (Are they predators whose effects increase diversity?);

(7) the human importance of the species (Are they endemics? Do they have unusual "charisma" for the public?);

(8) the vulnerability of the species (Is its distribution restricted? Is it subject to oil pollution? Are their preferred grounds near areas of high development potential?);

(9) the types of biological effects (e.g., oil contamination of plumage, PCB contamination of food chains); and

(10) whether the potential impacts are reversible or irreversible and to what degree.

b. To understand more of the effects of hunting on the behavior of marine birds and waterfowl on their breeding grounds, and to assess the effects on breeding performance of changes in behavior which result from human activities (such as hunting or studying the birds).

c. To understand the effects of the presence of predators (whether introduced or native) on breeding colonies in order to assess the importance of removing the predators or preventing their access to breeding grounds.

The Relation of the Products of Biological Research to Programs for Conservation of Marine Bird Resources

Although peaceful coexistence of wildlife populations and economic development are here assumed to be practical, some new social institutions are needed to control damaging activities of people during economic development. Human activities and industrial products which damage wildlife or their habitat must be identified, as must the space and resources which wildlife require for survival and health.

1. What seabird cliffs, islands, lagoons, wetlands, river mouths, and other habitat features are of first importance for breeding or for maintaining the populations? Some small areas of habitat are critical for the survival of some species during periods of stress. Those habitats need official recognition. Steps are needed to ensure that the habitats are maintained.

2. What physical expressions of economic development are of little, modest, or serious impact on wildlife and its habitat? These activities and constructions include harbors, storage sites, transshipment facilities, roads, pipelines, summer camps, and suburban or vacation developments.

3. What kinds of human activities will disturb, damage, or change the behavior or accessibility of wildlife? Many activities of one group of people have secondary effects which affect the enjoyment of resources for other groups. These include

a. gill netting for salmon, which may kill large numbers of murres and diving ducks;

b. release of predators on seabird nesting islands, which may kill adults or inhibit their feeding their young;

c. free running of pets (such as dogs and cats) over wetlands or wildlife habitat, because pets are predators and harass the wildlife which may be feeding;

d. flights of aircraft, especially helicopters, near or over seabird cliffs because such flights may cause serious damage to eggs and young;

e. hunting, because the game becomes timid and flees from those who might enjoy watching wildlife;

f. snow machines, because their presence is disagreeable to many and they provide easy access by which disturbing activities may reach into areas where wildlife would otherwise be undisturbed.

4. What limitations or alterations are needed in the existing legal institutions, such as the Marine Mammals Protection Act, the instruments implementing native land claims, the process of Alaska State lands withdrawal, the conditions for leasing State and Federal lands for development of mineral resources, and traditional rights of private property? All of these legal institutions are relevant to problems of wildlife survival and restoration, and within most of these institutions there exist conflicts between rights and benefits of special political interests and the husbanding of renewable common property resources.

Experience in Europe and in New England suggests that if reasonable limitations are set on human activities and that if adequate money charge is made against those who profit by economic development to defray full social costs, wildlife can continue to do well. In most cases where damage has occurred it is because those who administer the public institutions have failed to include consideration of the common property resources.

References

Ainley, D. G., and T. J. Lewis. 1974. The history of Farallon
Island marine bird populations. Condor 76(4):432-446.

Ames, P. L., and G. S. Mersereau. 1964. Some factors in the decline
of the osprey in Connecticut. Auk 81(2):173-185.

Andrewartha, H. G., and L. C. Birch. 1954. The distribution and
abundance of animals. Univ. Chicago Press, Chicago, Ill. 782 pp.

Ashmole, N. P. 1963. The regulation of numbers of tropical oceanic
birds. Ibis 103b:458-473.

Bailey, E. P., and G. H. Davenport. 1972. Die-off of common murres
on the Alaska Peninsula and Unimak Island. Condor 74(2):215-219.

Barry, T. W. 1967. Geese of the Anderson River delta, Northwest
Territories. Ph.D. Thesis. Univ. of Alberta, Edmonton, Alberta,
Canada. 176 pp.

Barry, T. W. 1968. Observations on natural mortality and native
use of eider ducks along the Beaufort Sea Coast. Can. Field-Nat.
82(2):140-144.

Bédard, J. 1969. Feeding of the least, crested, and parakeet
auklets around St. Lawrence Island, Alaska. Can. J. Zool.
47:1025-1050.

Belopol'skii, L. O. 1961. Ecology of sea bird colonies of the
Barents Sea. (Translated from Russian.) Israel Program for
Scientific Translations, Jerusalem. 346 pp.

Bent, A. C. 1919. Life histories of North American diving birds.
U.S. Natl. Mus. Bull. 107. 245 pp.

Bent, A. C. 1921. Life histories of North American gulls and terns.
U.S. Natl. Mus. Bull. 113. 345 pp.

Bent, A. C. 1922. Life histories of North American petrels and
pelicans and their allies. U.S. Natl. Mus. Bull. 121. 343 pp.

Bergman, G. 1957. Concerning the problem of mixed colonies: the
tufted duck and gulls. Die Vogelwarte 19(1):15-25.

Bertram, G. C. L., D. Lack, and B. B. Roberts. 1934. Notes on East
Greenland birds, with a discussion of the periodic non-breeding
among arctic birds. Ibis 13(4):816-831.

Brown, R. G. B. 1967. Breeding success and population growth
in a colony of herring and lesser black-backed gulls _Larus
argentatus_ and _L. fuscus_. Ibis 109(4):502-515.

Buckley, F. G., and P. A. Buckley. 1971. The breeding ecology
of royal terns _(Sterna [thalasseus] maxima maxima)_. Ibis
114(3):344-359.

Buckley, P. A., and F. G. Buckley. 1975. The significance of
dredge spoil islands to colonially nesting waterbirds in certain
national parks. Proc. Conf. on Management of Dredge Islands
in North Carolina Estuaries, May 1974. N.C. Sea Grant Publ.
UNC-SG-75-01, Raleigh.

Byrd, G. V., D. D. Gibson, and D. L. Johnson. 1974. The birds of
Adak, Alaska. Condor 76(3):288-300.

Coulson, J. C. 1966. The influence of the pair-bond and age on the
breeding biology of the kittiwake gull _Rissa tridactyla_. J.
Anim. Ecol. 35:269-279.

Coulson, J. C. 1968. Differences in the quality of birds nesting
in the centre and on the edges of a colony. Nature 217 (Feb.
3):478-479.

Coulson, J. C. 1974. Kittiwakes _Rissa tridactyla_. Pages 134-141
_in_ S. Cramp, W. R. P. Bourne, and D. Saunders, The Seabirds of
Britain and Ireland. Taplinger Publ. Co., Inc., New York. 287 pp.

Coulson, J. C., G. A. Potts, I. R. Deans, and S. M. Fraser. 1968.
Exceptional mortality of shags and other seabirds caused by
paralytic shellfish poison. Brit. Birds 61:381-404.

Coulson, J. C., and E. White. 1958. The effect of age on the
breeding biology of the kittiwake _Rissa tridactyla_. Ibis
100(1):40-51.

Coulson, J. C., and E. White. 1961. An analysis of the factors
influencing the clutch-size of the kittiwake. Proc. Zool. Soc.
Lond. 136:207-217.

Cramp, S., W. R. P. Bourne, and D. Saunders. 1974. The seabirds of
Britain and Ireland. Taplinger Publ. Co., Inc., New York. 287 pp.

Darwin, C. 1859. On the origin of species by means of natural
selection. John Murray, London. 490 pp.

Dement'ev, G. P., R. N. Meklenburtsev, A. M. Sudilovskya, and E. P.
Spangeberg. 1968. Birds of the Soviet Union. Vol. 2. (Translated
from Russian.) Israel Program for Scientific Translations,
Jerusalem. 553 pp.

Drost, R., E. Focke, and G. Freytag. 1961. Entwicklung und aufbau
einer population der Silbermöwe, _Larus argentatus argentatus_.
J. Ornithol. 102(4):404-429.

Drury, W. H. 1961. Observations on some breeding water birds on
Bylot Island. Can. Field-Nat. 75(2):84-101.

Drury, W. H. 1963. Results of a study of herring gull populations
and movements in southeastern New England. Pages 207-217 _in_
Colloque: Le problème des oiseaux sur les aérodromes (Nice,
1963). Inst. Nat. Rech. Agron., Paris.

Drury, W. H. 1973. Population changes in New England seabirds.
Bird-Banding 44(4):267-313.

Drury, W. H. 1974_a_. Population changes in New England seabirds.
Bird-Banding 45(1):1-15.

Drury, W. H. 1974_b_. Rare species. Biol. Conserv. 6(3):162-169.

Drury, W. H., and I. C. T. Nisbet. 1972. The importance of
movements in the biology of herring gulls in New England. Pages
173-212 _in_ Population Ecology of Migratory Birds: a Symposium.
U.S. Fish Wildl. Serv. Wildl. Res. Rep. 2. 278 pp.

Dutcher, W. 1901. Results of special protection to gulls and terns
obtained through the Thayer Fund. Auk 18(2):76-103.

Dutcher, W. 1904. Report of the A.O.U. Committee on the Protection
of North American Birds for the year 1903. Auk 21(suppl.):97-208.

Emison, W. B., F. S. L. Williamson, and C. M. White. 1971.
Geographical affinities and migrations of the avifauna on
Amchitka Island, Alaska. BioScience 21(12):627-631.

Fay, F. H., and T. J. Cade. 1959. An ecological analysis of the
avifauna of St. Lawrence Island, Alaska. Univ. Calif. Publ. Zool.
63:73-150.

Fernald, M. L. 1925. Persistence of plants in unglaciated areas of
boreal America. Mem. Am. Acad. Arts Sci. 15:239-342.

Fisher, J. 1952. The fulmar. Collins, London. 496 pp.

Fisher, J., and R. M. Lockley. 1954. Seabirds: An introduction
to the natural history of the seabirds of the North Atlantic.
Houghton Mifflin Co., Boston. 320 pp.

Fisher, J., and H. G. Vevers. 1943. The breeding distribution,
history and population of the North Atlantic gannet _(Sula
bassana)_. Part I. A history of the gannet's colonies, and the
census in 1939. J. Anim. Ecol. 12:173-213.

Fisher, J., and H. G. Vevers. 1944. The breeding distribution,
history and population of the North Atlantic gannet _(Sula
bassana)_. Part II. The changes in the world numbers of the
gannet in a century. J. Anim. Ecol. 13:49-62.

Flegg, J. J. M. 1972. The puffin on St. Kilda 1969-71. Bird Study
19:7-17.

Forbush, E. H. 1929. Birds of Massachusetts and other New England
States. Vol. I. Massachusetts Department of Agriculture, Boston.
481 pp.

Fordham, R. A. 1968. Dispersion and dispersal of the Dominican gull
in Wellington, New Zealand. Proc. New Zeal. Ecol. Soc. 15:40-50.

Fordham, R. A. 1970. Mortality and population change of Dominican
gulls in Wellington, New Zealand, with a statistical appendix by
R. M. Cormack. J. Anim. Ecol. 39(1):13-27.

Frazer-Darling, F. 1938. Bird flocks and the breeding cycle.
Cambridge Univ. Press, Cambridge, England. 124 pp.

Gilpin, M. E. 1975. Limit cycles in competition communities. Am.
Nat. 109:51-60.

Gromme, O. J. 1927. Some highlights on the faunal life of the
Alaskan Peninsula. Milw. Public Mus. Yearb. 7:30-45.

Hansen, H. A., and U. C. Nelson. 1957. Brant of the Bering
Sea--migration and mortality. Trans. N. Am. Wildl. Nat. Resour.
Conf. 22:237-256.

Harrington, B. A. 1974. Colony visitation behavior and breeding
ages of sooty terns _(Sterna fuscata)_. Bird-Banding
45(2):115-144.

Harris, M. P. 1964. Aspects of the breeding biology of the
gulls _Larus argentatus_, _L. fuscus_ and _L. marinus_. Ibis
106(4):432-456.

Harris, M. P. 1976. The present status of the puffin in Britain and
Ireland. Brit. Birds 69(7):239-264.

Harris, M. P., and W. J. Plumb. 1965. Experiments on the ability
of herring gulls _Larus argentatus_ and lesser black-backed
gulls _L. fuscus_ to raise larger than normal broods. Ibis
107(2):256-257.

Hickey, J. J., editor. 1969. Peregrine falcon populations--their
biology and decline. Univ. Wisconsin Press, Madison. 596 pp.

Hildén, O. 1965. Habitat selection in birds: a review. Ann. Zool.
Fenn. 2(1):53-75.

Holdgate, M. W., editor. 1971. The sea bird wreck in the Irish Sea,
autumn 1969. Nat. Environ. Res. Council Publ. Ser. C, No. 4. 17
pp.

Hultén, E. 1937. Outline of the history of arctic and boreal biota
during the Quaternary period. Bokforlags Aktiebolaget Thule,
Stockholm. 168 pp.

Hunt, G. L. 1972. Influence of food distribution and human
disturbance on the reproductive success of herring gulls. Ecology
53(6):1051-1061.

Jones, R. D. 1970. Reproductive success and age distribution of
black brant. J. Wildl. Manage. 34(2):328-333.

Kadlec, J. A. 1971. Effects of introducing foxes and raccoons on
herring gull colonies. J. Wildl. Manage. 35(4):625-636.

Kadlec, J. A., and W. H. Drury. 1968. Structure of the New England
herring gull population. Ecology 49(4):644-676.

Kenyon, K. W., and R. E. Phillips. 1965. Birds from the Pribilof
Islands and vicinity. Auk 82(4):624-635.

Klein, D. R. 1959. Saint Matthew Island reindeer-range study. U.S.
Fish Wildl. Serv., Spec. Sci. Rep.--Wildl. 43. 48 pp.

Kluyver, H. N. 1951. The population ecology of the great tit,
_Parus major_ L. Ardea 39:1-135.

Kluyver, H. N. 1966. Regulation of a bird population. Ostrich 38
(Suppl. 6):389-396.

Kluyver, H. N., and L. Tinbergen. 1953. Territory and the
regulation of density in titmice. Arch. Neér. Zool. 10:265-289.

Lack, D. 1948. Natural selection and family size in the starling.
Evolution 2:95-110.

Lack, D. 1954. The natural regulation of animal numbers. Clarendon
Press, Oxford. 343 pp.

Lack, D. 1964. A long-term study of the great tit _(Parus major)_.
J. Anim. Ecol. 33(Suppl.):159-173.

Lack, D. 1966. Population studies of birds. Clarendon Press,
Oxford. 341 pp.

Mackay, G. H. 1893. Observations on the breeding habits of _Larus
atricilla_ in Massachusetts. Auk 10(4):333-336.

Manuwal, D. A. 1974. Effects of territoriality on breeding in a
population of Cassin's auklet. Ecology 55(6):1399-1406.

May, R. M., and W. J. Leonard. 1975. Nonlinear aspects of
competition between species. SIAM J. Appl. Math. 29(2):243-253.

Mills, J. A. 1973. The influence of age and pair bond on the
breeding biology of the red-billed gull _Larus novaehollandiae
scopulinus_. J. Anim. Ecol. 42:147-162.

Murphy, R. C. 1936. Oceanic birds of South America, Vols. I and II.
Am. Mus. Nat. Hist., New York. 1245 pp.

Murray, M. D., and R. Carrick. 1964. Seasonal movements and
habitats of the silver gull _Larus novaehollandiae_ Stephens in
southeastern Australia. CSIRO Wildl. Res. 9:160-188.

Nelson, J. B. 1966. Population dynamics of the gannet at Bass Rock,
with comparative information for other Sulidae. J. Anim. Ecol.
35:443-470.

Nettleship, D. 1972. Breeding success of the common puffin
(_Fratercula arctica_ L.) on different habitats at Great Island,
Newfoundland. Ecol. Monogr. 42:239-268.

Nicholson, A. J. 1933. The balance of animal populations. J. Anim.
Ecol. 2:132-178.

Nisbet, I. C. T. 1972. Disaster year for terns. Man and Nature,
Dec. 1972:16-21.

Nisbet, I. C. T. 1973. Courtship-feeding, egg-size and breeding
success in common terns. Nature 241:141-142.

Nisbet, I. C. T., and W. H. Drury. 1972. Post-fledging survival in
herring gulls in relation to brood-size and date of hatching.
Bird-Banding 43(3):161-172.

Noble, G. K., and M. Würm. 1943. The social behavior of the
laughing gull. Ann. N. Y. Acad. Sci. 45:179-200.

Norton, A. H. 1921. Report of the field agent for Maine. Bird-Lore
23:355-356.

Norton, A. H. 1924. Notes on birds of the Knox County region. Maine
Nat. 4:35-39.

Palmer, R. S. 1949. Marine birds. Harvard Bull. Mus. Comp. Zool.
102. 656 pp.

Perrins, C. 1965. Population fluctuations and clutch-size in the
great tit, _Parus major_ L. J. Anim. Ecol. 34:601-647.

Pitelka, F. A., P. Q. Tomich, and G. W. Treichel. 1955. Ecological
relations of jaegers and owls as lemming predators near Barrow,
Alaska. Ecol. Monogr. 25:85-117.

Preble, E. A., and W. L. McAtee. 1923. A biological survey of the
Pribilof Islands, Alaska. U.S. Fish. Wildl. Serv., N. Am. Fauna
46:1-255.

Reed, A., and J. G. Cousineau. 1967. Epidemics involving the common
eider _(Somateria mollissima)_ at Ile Blanche, Quebec. Nat. Can.
(Que.) 94:327-334.

Sealy, S. G., F. H. Fay, J. Bédard, and M. D. F. Udvardy. 1971. New
records and zoogeographical notes on the birds of St. Lawrence
Island, Bering Sea. Condor 73(3):322-336.

Serventy, D. L. 1967. Aspects of the population ecology of the
short-tailed shearwater _Puffinus tenuirostris_. Proc. Int.
Ornithol. Congr. xiv:165-190.

Sladen, W. J. L. 1966. Additions to the avifauna of the Pribilof
Islands, Alaska, including five species new to North America. Auk
83:130-135.

Soots, R. F., Jr., and J. F. Parnell. 1975. Ecological succession
of breeding birds in relation to plant succession on dredge
islands in North Carolina estuaries. N.C. Sea Grant Publ.
UNC-SG-75-27, Raleigh.

Thompson, M. C., and R. L. DeLong. 1969. Birds new to North America
and the Pribilof Islands, Alaska. Auk 86(4):747-749.

Tickell, W. L. N. 1968. The biology of the great albatrosses,
_Diomedea exulans_ and _Diomedea epomorpha_. Antarct. Res. Ser.
12:1-55.

Tuck, L. M. 1960. The murres: their distribution, populations and
biology. Can. Wildl. Serv., Rep. Ser. No. 1. 260 pp.

Uspenski, A. M. 1956. The bird bazaars of Novaya Zemlya. U.S.S.R.
Acad. Sci., Moscow. (Translated from Russian.) Canad. Wildl.
Serv. Transl. Russian Game Rep. 4. 159 pp.

Vermeer, K. 1963. The breeding ecology of the glaucous-winged gull
_(Larus glaucescens)_ on Mandarte Island, B.C. Occas. Pap. B. C.
Prov. Mus. 13:1-104.

Wynne-Edwards, V. C. 1959. The control of population-density
through social behaviour: a hypothesis. Ibis 101:436-441.

Time-Energy Use and Life History Strategies of Northern Seabirds

by

Erica H. Dunn

Long Point Bird Observatory

Point Rowan, Ontario, Canada

Abstract

Time and energy budgets can be compared
among species of birds with very different
ecology as a way of summarizing differences
and as an approach to determining selective
pressures on each species. This paper reviews
time-energy use of northern seabirds. Energetic
cost of maintenance (basal metabolism,
thermoregulation, and procurement and
processing of food) depends largely on the
following factors: (1) small birds have higher
metabolic costs per unit size than do larger
ones; (2) body structure affects the cost of
locomotion as well as of food procurement; (3)
climate affects metabolic costs; and (4) food
availability and nutrition vary among food
types, and throughout the year within a food
type. Little is known of maintenance energetics
in seabirds. Time and energy allocations
to items beyond basic maintenance are also
compared. Patterns and costs of molt and
migration are known only in a general way, and
the variety of possible patterns suggests that
more research would be of value. Almost nothing
is known of location and daily activities of
seabirds outside the breeding season. The
review of breeding season activities is more
comprehensive, and stresses the variety of
factors known to affect timing, and the total
time devoted to and the energetic costs of
various aspects of reproduction. Some of these
factors are weather, year, geographic location,
feeding conditions, age, sex, and distance
of food source from the breeding colony.
Species characteristics such as clutch size,
egg and yolk size, developmental type, growth
rate, food type, and behavior combine with
environmental variables to make seabirds a very
diverse group in time and energy budgeting.
Time-energy studies and determination of
productive energy (energy remaining after
maintenance needs have been met) can be useful
in pinpointing those groups of birds and the
times of year when birds are most vulnerable
to environmental stress. Life history
considerations suggest that most seabirds are
adapted to low population turnover and would
not be able to recover quickly from sudden
increases in mortality.

Effective management of a population requires manipulation of the factors most critical in causing population increase or decrease. Deciding what these factors are and which are most suitable for effective manipulation is very difficult due to the complexity of life cycles and possible factors affecting demography. It takes a thorough knowledge of a species and of its relationships with the biotic and abiotic environment to make effective management decisions. The following review of seabird time and energy use is meant to emphasize the wide variation of species ecology within this avian group.

Time and energy patterning is being used as the basis of ecological comparison for the following reason. Any activity of an animal requires time and energy use; therefore, the patterning of use makes a common thread to which allocation to all activities in a bird's life cycle can be related. Time-energy patterns can be compared among birds with diverse food types, habitats, life cycles, and life expectancy, and therefore offer an opportunity for comparison not available through other kinds of analysis (King 1974).

The amounts of time and energy allocated by an organism to different aspects of survival and reproduction should be regarded as being molded by natural selection to optimize (not necessarily to maximize) lifetime reproductive output (Fisher 1958; Williams 1966; Schoener 1971). Thus, differences in time and energy use between species should reflect adaptation to different biotic and abiotic environments. By comparing time and energy use, one can gain insight into the selective pressures on each species and have a basis on which to compare complex ecology more meaningfully than if one listed other types of differences.

This review of time-energy use in northern seabirds cannot be comprehensive, largely because many of the necessary data are lacking. It stresses major areas of difference, however, and points out aspects about which little is yet known.

Cost of Living

Every animal must expend a basic amount of energy on normal maintenance, excluding activities normally allocated to a relatively narrow time span, such as reproduction. This "existence energy" expenditure consists of basal metabolism, thermoregulation, and the costs of gathering and processing food, and could also be referred to as the animal's basic "cost of living." In discussing the components of the cost of living, energy use is emphasized and time largely ignored--partly because metabolism occurs irrespective of time (it is not something the animal can turn off for a period) and partly because time use in normal maintenance and foraging has been little studied.

_Metabolism_

Basal metabolic rate (BMR) depends greatly on body size (Lasiewski and Dawson 1967; Zar 1968), and the costs per unit size are higher for a small bird than for a large one (Fig. 1). The BMR is somewhat lower in seabirds and other nonpasserines than in passerines of similar size (Dawson and Hudson 1970).

The relationship between BMR and body size is paralleled by that between size and other metabolic costs, such as for thermoregulation at a given temperature and for activity (Fig. 1; Kendeigh 1970; Tucker 1970; Schmidt-Nielsen 1972; Berger and Hart 1974; Calder 1974). Basal metabolic rate can therefore be used as an index of the overall cost of living as far as metabolic functions are concerned. Small birds must allocate a greater proportion of their energy resources than larger ones to merely staying alive, and have a higher cost of living.

The suggestion in Fig. 1 that it is easy to measure activity costs in a straightforward manner is misleading, because the figure represents measures taken under standard conditions. Factors known to affect the cost of flight, for example, include anatomical adaptations (such as wing loading and wing shape), the type of flight (ascending, descending, gliding), and the speed of flight (Fig. 2; Tucker 1969, 1974; Hainsworth and Wolf 1975). The cost of a series of short flights may be higher than that for a long one because of the extra energy required for takeoff and landing. A few estimates have been made for the cost of flight, mostly in birds moving almost constantly (Lasiewski 1963; Nisbet 1963; Tucker 1972, 1974; Utter and LeFebvre 1970; Berger and Hart 1974), but the methods may be inadequate for birds that fly short distances frequently.

Little work has been done on the cost of locomotion in seabirds: that of Eliassen (1963) on great black-backed gulls _(Larus marinus)_, Berger et al. (1970) on ring-billed gulls _(L. delawarensis)_, and Tucker (1972) on the laughing gull _(L. atricilla)_, and indirect calculations of soaring flight characteristics in albatrosses, _Diomedea_ spp. (Cone 1964), and the fulmar, _Fulmarus glacialis_ (Pennycuick 1960). Swimming has been shown to be more costly than flying in ducks (Schmidt-Nielsen 1972) and may be for seabirds as well. More energy is also probably used in underwater swimming than in flying.

The energetic costs of thermoregulation under natural conditions are not easy to estimate. Thermal energy is gained from and lost to the environment, and the degree of exchange depends not only on air temperature but also on metabolic rate, insulation, body temperature, posture, humidity, convection, and radiation. Radiation, in turn, depends on cloud cover, shade, and reflective and absorptive characteristics of the organism and of the environment (Porter and Gates 1969; Calder and King 1974). Most of these quantities are changing constantly, and insulation and metabolic rate may vary on a seasonal basis with acclimation (Dawson and Hudson 1970).

At present, no direct measurement technique exists for determining natural thermoregulatory costs, although a few estimates have been made (King 1974), including several for seabird nestlings (Dunn 1976_a_, 1976_b_ for double-crested cormorants, _Phalacrocorax auritus_, and for herring gulls, _Larus argentatus_). For most birds, the temperature environment actually faced over a year's time has never been measured, and for no bird has a full description of the complete thermal environment been made. It is clear that climate and degree of exposure are important elements in the basic cost of living, and that thermoregulatory costs average higher in small birds than in larger ones, but beyond that little information is available. Work on thermoregulatory costs of free-living chicks of two species of seabirds suggests that insulative properties can lead to marked differences in the metabolic costs of different species in an essentially identical environment (Dunn 1976_a_, 1976_b_).

_Food Procurement and Processing_

Gathering and processing food is another major component of the cost of living. Both the nutritional value of food and its availability (a rather vague term covering both abundance and ease of capture) are extremely diverse and variable, making estimations of foraging cost and benefit difficult (Ashmole 1971; Fisher 1972; Sealy 1975_a_).

Availability of food varies throughout the year, particularly in marine invertebrates that form the diet of many seabirds (e.g., Spaans 1971; Bédard 1969_a_). High arctic oceans have a very high peak of productivity in the summer, whereas the low arctic has a lower, but longer-lasting, peak (Ashmole 1971). Fish stocks increase in summer as well (Snow 1960; Pearson 1968; Sealy 1975_a_), and decline or disperse in autumn (Potts 1968). Catch-ability may also differ widely from year to year (e.g., E. K. Dunn 1973).

Marine foods are likely to have a patchy distribution, which may make food stocks difficult to locate, even in times of abundance (Ashmole 1971; Sealy 1975_a_). Birds in localities with low food abundance frequently show alterations in time and pattern of foraging, sometimes even changing diets (Cramp 1972; Henderson 1972; Hunt 1972; Lemmetyinen 1972). The time and energy expended in finding and capturing food by different seabird species must vary widely according to the form of foraging used: plunge-diving, beach scavenging, aerial robbing, underwater pursuit, and so on. Even when different species have traveled the same distance to an identical food stock, therefore, the costs of procurement differ.

Time and energy spent foraging depends not only on abundance and ease of capture, but also on nutritional return, and on the age and size of the bird. Fig. 3 shows that the smaller species in a seabird community may spend the most time foraging. Even though this illustration is taken from the breeding season when food demands of the young must be taken into account, it suggests a difference based on cost of living according to size.