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.