POPULATION DYNAMICS IN NORTHERN MARINE BIRDS 



131 



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 natu- 

 ral 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). Dif- 

 ferences in breeding success also occur be- 

 tween 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 is- 

 lands in the Gulf of Maine (Drury 1963; Kad- 

 lec and Drury 1968; Hunt 1972). The popula- 

 tions of successful colonies grow while the 

 numbers of unsuccessful colonies decline, 

 even during a period of general population in- 

 crease (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 pro- 

 ductive 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 in- 

 dividuals to disperse while the majority of 

 individuals remain loyal to a colony can be 



considered a major mechanism of population 

 maintenance. If conditions deteriorate seri- 

 ously at one place so that the local popula- 

 tions decline or disappear, dispersal from 

 other centers can be expected to repopulate 

 the area as soon as local conditions again be- 

 come suitable. This subject has been treated 

 in more detail by Drury and Nisbet (1972) and 

 Drury (19746). 



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 geographi- 

 cal 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 (Pants bicolor) have settled in south- 

 eastern 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 spar- 



