Fig. 10. Growth rate as a function of body weight. Growth rate T10-90 represents the number of days to grow from 10% to 90% of asymptotic weight (Ricklefs 1968). Data from Ricklefs (1968, 1973), E. H. Dunn (1973), and Sealy (1973b). Solid circles and regression line, altricial birds; solid triangles, semiprecocial birds except for seabirds with one-egg clutches; open circles, precocial shorebirds; open triangles, precocial ducks, rails, and gallinaceous birds; solid squares, alcids with one-egg clutches; and open squares, northern petrels, gannet, and Manx shearwater (Puffinus puffinus).
Daily time budgets of adults raising nestlings also vary widely, depending on the amount of brooding required, food requirements of the young, and foraging costs (which differ in the breeding season from those at other times of the year).
Nestlings have imperfect control of body temperature at hatching (Fig. 11) and develop this capacity only gradually. Altricial birds are hatched at a particularly undeveloped stage; e.g., double-crested cormorants attain reasonable control of body temperature in moderate ambient temperatures only after about 14 days (Fig. 11; Table 5). Semiprecocial seabirds, which are more fully developed physically at hatching, attain control of body temperature much sooner, in a matter of several days, and precocial eiders can thermoregulate within a few hours after hatching (Table 5).
Until the age of temperature control, nestlings must be brooded almost constantly, and occasional brooding takes place for some time afterward, especially in severe weather, in all species studied (Tinbergen 1960; Belopol'skii 1961; Weaver 1970; Dunn 1976a, 1976b). Thermoregulatory capabilities in cold weather are better in ducklings of species nesting at high latitudes than at lower ones (Koskimies and Lahti 1964), and the same may be true of gull species (Dawson et al. 1972). The cooling mechanisms of double-crested cormorants are better than in the more northerly distributed pelagic cormorant, Phalacrocorax pelagicus (Lasiewski and Snyder 1969). Thus, variation in cost of thermoregulation due to different environments may be reduced through adaptation.
Food requirements of the chick depend on 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 factors vary according to developmental type, and comparison with budgets for nonseabird species suggests wide variation within developmental types according to the particular adaptations of each species to its own environment (E. H. Dunn 1973).
Fig. 11. Development of thermoregulatory capabilities in nestling double-crested cormorants. From Dunn (1976a). Ages at right refer also to corresponding oxygen consumption data on the left. Thin diagonal lines show equality between body and air temperature. All data taken after 2 h of exposure.
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, however, particularly in requirements for activity and growth, and adults can frequently raise young successfully without providing optimum amounts of food (Spaans 1971; Kadlec et al. 1969; LeCroy and Collins 1972; Lemmetyinen 1972; Cody 1973; E. H. Dunn and I. L. Brisbin, manuscript in preparation). Studies of double-crested cormorants by Dunn (1975b) 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 approximately double the amount of food gathered each day over the amount gathered by nonbreeders. This relation does not imply, however, that the time and energy allocation of the adults would be the same for the two species.
