130 



W. H. DRURY 



manifest in titmice, grouse, or rabbits. Near 

 failure of reproduction during a breeding sea- 

 son among arctic seabirds at Bear Island was 

 reported by Bertram et al. (1934). Many simi- 

 lar observations have been made since then: 

 Pitelka et al. (1955) reported such a case 

 among skuas and gulls at Point Barrow, 

 Drury (1961) for greater snow geese (Chen 

 cerulescens atlantica) at Bylot Island, Jones 

 (1970) for black brant gathering at Isambek 

 Lagoon on the Alaska Peninsula, and D. A. 

 Snarski (personal communication) for kitti- 

 wakes at Cook Inlet. Reproductive failure can 

 sometimes be chronic, as observed by Nisbet 

 (1972) for terns at Cape Cod, Massachusetts, 

 or by Drury (1963) and Hunt (1972) for 

 herring gulls on the outer islands on the coast 

 of Maine. 



When reproductive failure becomes chronic 

 as observed on peregrine falcons (Falco pere- 

 grinus) by Hickey (1969) and in ospreys 

 (Pandion haliaetus) by Ames and Mersereau 

 (1964), the population of adults may hold on 

 for a number of years without evident decline. 

 Damage to the structure of the whole popula- 

 tion may be serious before any numerical re- 

 sults are evident. 



Although there may not be intensive com- 

 petition for food in the habitat away from 

 breeding colonies, there is intense competition 

 for food and breeding sites at and around the 

 colonies. Hence age and previous experience 

 in seabirds assume importance in establishing 

 territory and in breeding success. Associated 

 with this is the tendency for immature birds 

 to delay breeding until they are several years 

 old and for the immatures to remain on feed- 

 ing grounds at some distance from the colo- 

 nies. In some cases young birds may "hang 

 around" breeding colonies and even feed some 

 of the young. When young birds do first breed 

 they usually lay smaller clutches and raise 

 fewer young than do older birds. The impor- 

 tance of age and experience upon breeding 

 success has been well documented for kitti- 

 wakes (Coulson 1966) and red-billed gulls 

 (Mills 1973). 



The fundamental biological importance of 

 this delayed maturity seems to be emphasized 

 by the persistence for several years of imma- 

 ture plumages, so clearly identifiable that 

 even a human observer can recognize the age 

 of an individual. One assumes such an evident 



feature must have adaptive significance. 

 Wintering Grounds 



When colonial nesting seabirds leave their 

 breeding islands for their wintering grounds, 

 their identification with that island is lost as 

 far as population effects are concerned, be- 

 cause birds from many colonies mingle on the 

 wintering grounds. Major mortality takes 

 place on the wintering grounds and must 

 therefore act on the species population as a 

 whole rather than differentially on individuals 

 associated with especially dense colonies. 

 Such a direct relation between colony density 

 and mortality would be necessary for density- 

 dependent mortality to regulate the number 

 of birds on a breeding colony. Conversely, one 

 cannot expect that all colonies will decrease 

 equally because mortality should be equally 

 distributed if all the population gathers on a 

 common wintering ground. Thus density-de- 

 pendence acts only in a very general way upon 

 the sum of animals considered as an abstract 

 entity the population. 



In fact, on the wintering grounds, as shown 

 by a graph of numbers of gulls reported on 

 Christmas Counts on Cape Cod, Massachu- 

 setts (Kadlec and Drury 1968), herring gulls 

 are very responsive to local conditions and 

 move several tens of miles to gather at favor- 

 able feeding sites. An aerial survey of the 

 gulls on the East Coast of the United States 

 (Kadlec and Drury 1968) showed that more 

 than half of the gulls were gathered near 

 major food sources in large metropolitan dis- 

 tricts. Most of the remainder were gathered 

 near small fishing ports. Very few were 

 scattered along the shoreline in what one as- 

 sumes is the traditional gull habitat. Later 

 analyses of the relation between the distribu- 

 tion of banding recoveries of birds in their 

 first winter and the distribution of immatures 

 as found on this winter census (Drury and 

 Nisbet 1972) suggested that proportionately 

 more first-year gulls died in those areas where 

 the birds were sparsely distributed than died 

 in the crowded metropolitan areas. 



These results suggest both that there is not 

 a direct feedback between reproductive rate 

 and mortality, and that mortality may even 

 be inversely density-dependent on wintering 

 grounds. This last runs counter to traditional 



