TIME-ENERGY USE AND LIFE HISTORY STRATEGIES 



153 



(Gross 1935), whereas herring gulls cover 

 their eggs 98% of the time (Drent 1970). 



The sexes share in incubation in most sea- 

 birds (Snow 1960; Drent 1965, 1970; Bedard 

 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 en- 

 tire 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 de- 

 velopment 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 (Ken- 

 deigh 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 de- 

 gree during incubation. 



In spite of the lack of quantitative data, one 

 can surmise that the cost of incubation varies 

 among sea birds. Precocial and semiprecocial 

 birds tend to have a larger clutch weight rela- 

 tive to body weight than to 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 de- 

 velopmental 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 nor- 

 mally remain at the nest until 75-90% grown, 

 but certain alcids leave much sooner well be- 

 fore the young can fly. 



SHAG 



MEW GULL 



BLACK GUILLEMOT 



ARCTIC TERN 



GREAT BLACK-BACKED GULL 



PARASITIC JAEGER 



BLACK-LEGGED KITTIWAKE 



HERRING GULL 



COMMON PUFFIN 



DOVEKIE 



COMMON MURRE 



THICK-BILLED MURRE 



COMMON EIDER 



50 100 



PERCENT TOTAL GROWTH 



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 unaf- 

 fected by growth rate (which is typically very 

 slow), since they leave soon after hatching. 

 The nestling period of semiprecocial and altri- 

 cial 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 develop- 

 mental type, as some semiprecocial species 

 grow rather slowly. Certain seabirds with 

 clutches of one egg grow particularly slowly 

 (petrels, some alcids, sulids). Several other al- 

 cids with single-egg clutches, however, grow 



