TIME-ENERGY USE AND LIFE HISTORY STRATEGIES 



155 



2.0 



16-17 OAYS-o 

 18-35 DAYS - 



40 



20 



30 40 10 20 30 



AMBIENT TEMPERATURE- *C 



Fig. 11. Development of thermoregulatory capabili- 

 ties in nestling double-crested cormorants. From 

 Dunn (1976a). Ages at right refer also to corre- 

 sponding oxygen consumption data on the left. 

 Thin diagonal lines show equality between body 

 and air temperature. All data taken after 2 h of 

 exposure. 



growth rate, amount of fat deposition, cost of 

 thermoregulation, degree of activity and 

 other factors (E. H. Dunn 1973). Estimated 

 energy budgets for nestling double-crested 

 cormorants and herring gulls in the same year 

 and locality (Fig. 12) indicate that these fac- 

 tors vary according to developmental type, 

 and comparison with budgets for nonseabird 

 species suggests wide variation within de- 

 velopmental types according to the particular 

 adaptations of each species to its own envi- 

 ronment (E. H. Dunn 1973). 



Thus, the energy demands of nestlings are 

 not easy to predict. Brood size differences 

 multiply variation in food demand on adults 

 (except in precocial birds whose young feed 

 themselves). Energy demands are labile, how- 

 ever, particularly in requirements for activity 

 and growth, and adults can frequently raise 

 young successfully without providing opti- 

 mum amounts of food (Spaans 1971; Kadlec 



CORMORANT 



Fig. 12. Energy budgets of nestling double-crested 

 cormorants and herring gulls. Data from E. H. 

 Dunn (1973) and Brisbin (1965). 



et al. 1969; LeCroy and Collins 1972; Lemme- 

 tyinen 1972; Cody 1973; E. H. Dunn and I. L. 

 Brisbin, manuscript in preparation). Studies 

 of double-crested cormorants by Dunn (19756) 

 and pigeon guillemots (Cepphus columba) by 

 Koelink (1972) have suggested that each adult 

 providing optimum amounts of food to a 

 normal-sized brood would have to approxi- 

 mately double the amount of food gathered 

 each day over the amount gathered by non- 

 breeders. This relation does not imply, how- 

 ever, that the time and energy allocation of 

 the adults would be the same for the two 

 species. 



Cost-benefit ratios of food gathering in the 

 nestling period differ from those at other 

 times. Besides facing increased food demands, 

 costs of delivery to the nest, and changes in 

 food availability, the parents' choice of foods 

 is constrained by the need to forage within 

 reasonable commuting distance of the nest 

 and perhaps by concentrated competition 

 with conspecifics and other seabird species. In 

 addition, small nestlings are frequently un- 

 able to eat foods normally eaten by adults 

 (Drent 1965; personal observation). In the 

 face of these constraints, adults often shift 



