Each species has a different incubation rhythm. In birds in which the sexes share in incubation, the sexes exchange places at intervals that differ widely among different birds: several hours in lariforms and some alcids (Drent 1965; Lack 1968; Preston 1968; Drent 1970); about 4 h in shags (Snow 1963); up to 11 h in the ivory gull (Bateson and Plowright 1959); up to 24 h in certain other alcids (Manuwal 1974a; Sealy 1975a); 33 h (on the average) in common puffins (Myrberget 1962); 72 h in ancient murrelets, Synthliboramphus antiquus (Sealy 1975a); and 96 h in Leach's storm-petrel (Gross 1935). Degree of attentiveness once a bird is on the nest also varies. Petrels may leave the egg for several days (Gross 1935), whereas herring gulls cover their eggs 98% of the time (Drent 1970).
The sexes share in incubation in most seabirds (Snow 1960; Drent 1965, 1970; Bédard 1969a), 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 1974a). 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.
Raising Nestlings
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.
The stage of growth attained when birds leave the nest varies considerably (Fig. 9). Precocial eiders leave the nest within a day of hatching, whereas altricial shags remain until completely grown. The young of semiprecocial species, on the other hand, leave the nest at all stages between these extremes. Larids normally remain at the nest until 75-90% grown, but certain alcids leave much sooner—well before the young can fly.
Fig. 9. Percentage of total growth completed in the egg (shaded bar at left), at the nest site (open bar), and after nest-leaving (shaded bar at right) in various northern seabirds. From Belopol'skii (1961).
Growth rate depends both on body size and developmental type (Fig. 10). The length of stay at the nest for precocial young is unaffected by growth rate (which is typically very slow), since they leave soon after hatching. The nestling period of semiprecocial and altricial seabirds is, however, affected by the rate at which the young grow to the nest-leaving stage. This depends mainly on body size (Fig. 10) and to a certain degree on developmental type, as some semiprecocial species grow rather slowly. Certain seabirds with clutches of one egg grow particularly slowly (petrels, some alcids, sulids). Several other alcids with single-egg clutches, however, grow at rates normal for semiprecocial chicks (Fig. 10). Very slow growth may be related to food stress (Lack 1968; Ricklefs 1968) or to reduction of reproductive effort in the adults (discussed later). Contrary to Cody (1973), slow growth in alcids does not correlate to the distance adults must commute for food. (Cody tried to directly compare growth in birds of different sizes.) Chicks in nocturnal species, however, tend to have slow growth rates (Sealy 1973b).