TIME-ENERGY USE AND LIFE HISTORY STRATEGIES 



159 



JAN FEB MAR APR MAY JUN 

 RED-WINGED BLACKBIRDS 



JAN FEB MAR APR MAY JUN 

 TRICOLORED BLACKBIRDS 



Fig. 16. Time and energy budgets of male and female red-winged (Agelaius phoeniceus) and tricolored (A. tri- 

 color} blackbirds in the breeding season. From Orians (1961). Dotted lines show male (M) activity, dashed 

 lines show female (F) activity, and solid lines show shared activities. 



are good, however, it may be more advan- 

 tageous to reduce annual reproductive effort 

 and allocate resources to other functions. 



Seabirds are generally long-lived, have 

 small clutches, and generally delay first 

 breeding until at least the 2nd year, and 

 usually longer (Table 6). Phalaropes seem to 

 differ from this pattern (Hilden and Vuolanto 

 1972; Howe 1975). Several ecological factors 

 (not entirely independent) are believed to con- 

 tribute to the evolution of the long life and 

 low reproductive effort pattern favored by 

 seabirds. 



First, if population size is determined 

 largely by density-dependent mortality, indi- 



viduals may be favored that allocate re- 

 sources to attaining longer life (and more 

 chances to reproduce) or insuring greater 

 chances of survival of their offspring (Murphy 

 1968; Hairston et al. 1970). Density-inde- 

 pendent mortality, on the other hand, is so un- 

 predictable that there is no advantage in allo- 

 cating resources toward protection against it 

 (Gadgil and Solbrig 1972). 



Two factors closely linked with density-de- 

 pendence are high levels of competition, and 

 perennial difficulties in obtaining food. In 

 adapting to these difficulties, a bird may be 

 selected which develops more efficient forag- 

 ing techniques, wider dispersal, or better abili- 



