A bird must satisfy not only energetic needs, but also nutritional requirements. Fish are high in protein (Table 1), but what little is known of marine invertebrates suggests a low proportion of protein in relation to total bulk. Protein is vital to growth of nestlings (Fisher 1972; Lemmetyinen 1972), and 4-8% protein in the diet seems to be required for minimal maintenance of adults (Martin 1968; Fisher 1972). Some seabirds (such as puffins, Fratercula) that eat a varied diet raise their young exclusively on fish (Bédard 1969b; Nettleship 1972; Sealy 1973a).
Other aspects of nutrition, such as vitamins and minerals, are also important to avian health (Brisbin 1965; Fisher 1972). To further complicate matters, nutritional values vary with season, as do birds' requirements for them (Myton and Ficken 1967; Moss 1972). Adults must adjust their time and energy allocation to foraging to optimize not only energetic, but also nutritional return.
Optimal time and energy allocation has been studied in theory (Orians 1971; Schoener 1971; Pulliam 1974; Katz 1974) and some direct observations have been carried out, largely on seedeaters (Bookhout 1958; Myton and Ficken 1967; Royama 1970; Moss 1972; Willson 1971; Willson and Harmeson 1973). The direct studies, in particular, point out the basic importance of studying cost-benefit ratios by interrelating complex factors of food availability, searching patterns, and type, size, and caloric and nutritional value of foods.
Time-energy Use Beyond the Cost of Living
This section concerns variation in time and energy allocated by seabirds to activities above and beyond the cost of living—particularly to migration, molt, and reproduction. Allocation to such items as avoidance of predation and competition is not considered here, because they are not readily analyzed as activities to which time and energy are devoted in a specific part of the annual cycle.
The previous discussion dwelt on energy considerations and could have referred to almost any group of birds. The following treatment centers on time use of northern seabirds. Little is known of energetics beyond the cost of living, although estimates have been made for certain aspects of migration, molt, and reproduction (Nisbet 1963; Hussell 1969; Hart and Berger 1972; Payne 1972; Ricklefs 1974). Essentially nothing is known, however, of the relationship of such costs to the amount of energy available to the bird once basic maintenance costs have been met (productive energy). Because such complete data are not available, the following account dwells largely on variation in timing and total time devoted to activities beyond basic maintenance.
Fig. 4. Typical patterns of generalized annual cycles in reproduction, migration, and wing molt in northern seabirds. Solid line shows reproductive season, dotted line the period of migration or dispersion, and dashed line the period of annual primary molt. Data from Dorst (1961), Stresemann and Stresemann (1966), and Ashmole (1971).