POPULATION DYNAMICS IN NORTHERN MARINE BIRDS 



133 



centage 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), At- 

 lantic puffins by Nettleship (1972), and 

 Cassin's auklets (Ptychoramphus aleutica) by 

 Manuwal (1974), and the practice of eider 

 "farming" in Iceland indicate that the num- 

 ber 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 dump- 

 ing 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 inadvis- 

 able 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 in- 

 crease. Fox predation may have important ef- 

 fects over most ranges of prey density be- 

 cause 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 ef- 

 fective programs of conservation should in- 

 clude guarantees that a number of colony 

 sites be available in as widely dispersed a pat- 

 tern as possible. Each productive feeding 

 ground should, if possible, have several 

 colony sites available. 



We have argued elsewhere (Drury and Nis- 

 bet 1972; Dri&y 1974a) that one of the chief de- 

 fenses 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 af- 

 fect 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 in- 

 sured against extinction. This, of course, sug- 

 gests 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 "conserva- 

 tive" species, such as those endemic to the 

 Bering Sea region (whose dispersal and colo- 

 nizing mechanisms seem to be poorly de- 

 veloped), are especially vulnerable to the ef- 

 fects 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 manage- 

 ment techniques will be built upon ensuring 

 the health of colonies and the associated feed- 

 ing areas at which reproductive success is 

 high enough to "export" young. Thus it is 

 useful to identify those colonies which are ex- 

 porting 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 de- 

 crease, as in the case of "guano birds" during 

 El Nino years in the Peru Current. 



Because centers of abundance of marine 

 birds shift (Fisher 1952; Drury 1963, 1974a), 

 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 sea- 

 bird sanctuary through the long corridors of 

 the distant government bureaucracies in Ot- 

 tawa, 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. 



