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 1974a) 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.