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).