158 



E. H. DUNN 



100 



DRINK. BATHE 



JFMAMJ JAS OND 

 MONTHS OF YEAR 



Fig. 15. Time budget of male (upper panel) and fe- 

 male (lower panel) yellow-billed magpies through- 

 out the year. From Verbeek (1972). Nonlabeled 

 portions in each graph correspond to labeled sec- 

 tions in the other. 



1965; Schartz and Zimmerman 1971; Stiles 

 1971; Wolf and Hainsworth 1971; Smith 1973; 

 Utter and LeFebvre 1973). 



Time-energy budget analysis can be useful 

 in determining the leeway a bird has in surviv- 

 ing unusual stress at different times of the 

 year. For example, a study by Feare et al. 

 (1974) showed that rooks (Corvus frugilegus) 

 in the dry part of the summer spent 90% of 

 15 h of daylight to collect 150 kcal of food 

 energy. In winter, foraging in snow, the same 

 birds were able to collect 240 kcal of food in 

 only 30% of a 10-h day. This suggests that 

 rooks would be far more vulnerable to unex- 

 pected periods of stress in late summer than 

 in winter. Such information would clearly be 

 useful in making management decisions. 



A more precise measure of vulnerability, al- 

 though much more difficult to determine, is 



that of productive energy the amount of 

 caloric intake left over after the birds' cost of 

 living (metabolic functions and procurement 

 and processing of food) have been accounted 

 for. Costs are highest when temperatures are 

 extremely hot or cold or when food is most dif- 

 ficult to obtain. Productive energy is highest 

 in summer (Kendeigh 1972), and that is pre- 

 sumably why reproduction normally takes 

 place then. It is unknown whether birds are 

 more vulnerable to time and energy shortages 

 in the harder nonbreeding season or in the 

 breeding season after the extra demands of re- 

 production have been accounted for. Vulner- 

 ability may also differ between sexes and 

 among age groups. 



Time-energy studies, although useful in 

 comparing ecology, determining vulner- 

 ability, and cataloging location of birds, do 

 have limitations. Careful studies are time-con- 

 suming and are not the best approach to de- 

 termining key factors influencing population 

 increase or decrease. Even when different 

 kinds of data are being sought, however, it is 

 worthwhile keeping the time-energy frame- 

 work in mind as a "big picture" into which 

 other facts can be fitted and their significance 

 considered. 



Life History Strategies 



The study of life history strategies is 

 largely theoretical, and in the following dis- 

 cussion I do not comment on current theoreti- 

 cal arguments. On the other hand, life history 

 strategies can be regarded as time and energy 

 allocation on a grand scale, and it therefore 

 seems appropriate to look briefly at their im- 

 plications for seabird management. 



Annual reproduction evidently has a nega- 

 tive effect on resources remaining for other 

 functions, and may reduce the chances for an 

 organism to reproduce again in a later season 

 (Cody 1966, 1971; Williams 1966; Gadgil and 

 Bossert 1970; Gadgil and Solbrig 1972; 

 Hussell 1972; Trivers 1972; Calow 1973). If 

 the chances of survival to another breeding 

 season are small, the selective advantage lies 

 with the bird putting the most effort into 

 early reproduction, in spite of its negative ef- 

 fects on survival, because future chances of 

 reproduction are small. If chances of survival 



