TIME-ENERGY USE AND LIFE HISTORY STRATEGIES 



161 



complex and poorly understood, and simple 

 formulas should not be expected to apply to 

 all situations (Wilbur et al. 1974). 



In the framework of life-history strategies, 

 small clutch sizes and slow growth rates ex- 

 hibited by some seabirds can be explained as 

 adaptive reductions in annual reproductive ef- 

 fort, rather than as responses to immediate 

 food shortages. Arguments for this view are 

 presented on theoretical grounds (Dunn 1973) 

 and by the fact that many seabirds are able to 

 raise larger than normal broods in certain 

 situations (Vermeer 1963; Nelson 1964; 

 Harris 1970; Hussell 1972; Ward 1972; Cork- 

 hill 1973). In addition, seabirds with particu- 

 larly slow growth rates all grow at about the 

 same rate, regardless of body size (contrary to 

 the situation in other birds). This suggests 

 that low growth rates do not reflect varia- 

 tions in feeding abilities among species (Rick- 

 lefs 1968). 



Several conclusions relating to manage- 

 ment of seabird populations can be drawn 

 from the above discussion. First, if population 

 size is determined largely by density-de- 

 pendent factors, the birds are not adapted to 

 precipitous and unexpected declines in popu- 

 lation levels. Because there is low annual re- 

 productive effort geared to a world in which 

 there is slow turnover in population, seabirds 

 are not able to rebound quickly from 

 disasters. Provision of excess food should not 

 be expected to improve breeding performance, 

 at least in experienced birds. 



Second, because seabirds are able to repro- 

 duce in many different seasons and are 

 adapted to a low reproductive effort within a 

 given season, one should expect them to be 

 easily disturbed and to fail to complete the re- 

 productive cycle during any given breeding 

 attempt. A few indications of such failures 

 have already been observed (Erskine 1972; 

 Manuwal 1974a; Nettleship 1975). 



Again, the tentative nature of this discus- 

 sion should be emphasized, and conclusions 

 drawn from it may not apply equally to all 

 seabird species. 



Conclusions 



In this discussion I have tried to emphasize 

 the variety of factors affecting seabird life 



cycles and the diverse responses among dif- 

 ferent species to their environment. The main 

 conclusion I stress is that each species (and 

 age group and sex within that species) has a 

 different vulnerability to stress, which may be 

 most severe at different times of the year for 

 each group. To determine these periods of 

 stress, researchers may find a time-energy ap- 

 proach to be useful. 



As for northwestern North American sea- 

 birds in particular, ignorance is vast. Twelve 

 years ago, Bourne (1963:846) noted the follow- 

 ing needs in seabird research (among others): 

 "The investigation of seabird biology has 

 been reduced to a routine, but there is a great 

 need for more study of some other aspects of 

 the life or annual cycle, including events in the 

 period immediately after fledging, and be- 

 haviour and survival in the immature period 

 and outside the breeding season. Much more 

 accurate information is needed about breed- 

 ing distribution and seasons in many parts of 

 the world, about molting seasons and ranges 

 in most parts, and the distribution of birds of 

 different age groups during these periods in 

 practically all areas." 



Since the time of Bourne's remarks, a num- 

 ber of excellent studies have provided data on 

 the breeding biology of certain northwestern 

 seabird species. Scientists remain largely 

 ignorant, however, about where birds of dif- 

 ferent age groups are located throughout the 

 year. Such knowledge is necessary for effec- 

 tive protection and is basic to understanding 

 population dynamics, even if it does not eluci- 

 date causes. Studies of timing of annual 

 cycles and movements should be carried out 

 hand in hand with resource analysis not just 

 finding what birds eat, but discovering where 

 the food is at what times, how hard it is to 

 catch, and what the nutritional return is. 

 Much careful field work must be done before 

 effective management of most of our north- 

 western seabirds can become a reality. 



References 



Andersson, M. 1973. Behaviour of the pomarine 

 skua Stercorarius pomarinus Temm. with com- 

 parative remarks on Stercorariinae. Ornis Scand. 

 4:1-16. 



Ashmole, N. P. 1971. Seabird ecology and the 

 marine environment. Pages 223-286 in D. S. 



