POPULATION DYNAMICS IN NORTHERN MARINE BIRDS 



129 



Breeding 



Ashmole (1963) suggested that the clutch 

 size of some oceanic birds is small and colo- 

 nies occupy only part of the available habitat 

 because food resources within efficient com- 

 muting distance of the breeding site are 

 limited. We can see this effect in the usual fail- 

 ure 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). 



Ashmole (1963) suggested that during the 

 course of the breeding season the birds ex- 

 haust the available food supply. The validity 

 of this suggestion is reflected in the long dis- 

 tances 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 novae-hollandiae 

 (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 cur- 

 rents 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 elimi- 



nated. An exception to this is the food sub- 

 sidy 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 ex- 

 tremes little suspected until recently. Mortali- 

 ties 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. Be- 

 cause the titmice are permanent residents, 

 they occupy all of the available habitat 

 throughout the year. Hence titmice suffer in- 

 tense intraspecific competition, which 

 shortens the survival of adults. Kluyver 's ex- 

 periments (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 short- 

 ened 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 ob- 

 servable immediate effects that would be 



