The sexes share in incubation in most seabirds (Snow 1960; Drent 1965, 1970; Bédard 1969_a_), although females frequently take on the greater role (Belopol'skii 1961). Only male phalaropes incubate the eggs, and only female eiders. Eider hens do not feed during the entire incubation period (25 days) and leave the nest only for short periods of about 10 min (Belopol'skii 1961; Schamel 1974).
Several methods exist for calculating the amount of heat input necessary for normal development of a clutch of eggs (Ricklefs 1974). There is controversy, however, as to whether an adult can provide this warmth from excess body heat lost during the course of normal metabolism or whether the adult must raise its metabolic level to produce extra heat (Kendeigh 1973; King 1973; Ricklefs 1974). Several studies of incubating birds suggest that, in at least some situations, adults need not raise metabolic levels, but in others (large clutch, severe weather), they probably do (Ricklefs 1974). Drent (1972) estimated that herring gulls raise metabolic levels to a significant degree during incubation.
In spite of the lack of quantitative data, one can surmise that the cost of incubation varies among seabirds. Precocial and semiprecocial birds tend to have a larger clutch weight relative to body weight than do altricial birds (Fig. 8; Lack 1968), and therefore require greater heat input to the eggs. These costs may be reduced by heavily insulating the nest (e.g., eiders), or by nesting in burrows, which have much more moderate and even climates than do external nests (Richardson 1961; Manuwal 1974_a_). Other semiprecocial species, however, such as the murre, may sometimes lay eggs directly on snow or ice (Belopol'skii 1961)--presumably at increased incubation costs. Lastly, certain species incubate eggs with their feet (e.g., cormorants), rather than develop featherless brood patches. There are no measurements of comparative heat flow from feet versus brood patches.
The length of the nestling period (hatching until departure from the nest) varies greatly among northern seabirds (Fig. 6). Nestling period depends on the stage of growth at which the young leave the nest and the rate at which they attain that stage. Growth rate in turn depends largely on body size and developmental type.