TIME-ENERGY USE AND LIFE HISTORY STRATEGIES 



151 



Contrary to the situation in passerines, sea- 

 birds tend to lay their clutches with relatively 

 large time intervals between eggs. Eggs may 

 be laid every 2nd or 3rd day in alcids, larids, 

 sternids, stercorariids, and phalacrocoracids 

 (Lack 1968), but every day in phalaropes 

 (Howe 1975). Inasmuch as clutch size in 

 northern seabirds varies from one to five or 

 six, the length of the laying period varies 

 widely among species. 



Energetic costs of egg laying depend on the 

 actual caloric content of the egg and the speed 

 with which the ova are developed (Ricklefs 

 1974). The energy in the egg is contained 

 mainly in the yolk, and yolk size depends 

 largely on the developmental pattern shown 

 by the young after hatching (Table 3). Pre- 

 cocial chicks are hatched at a relatively ad- 

 vanced stage, are covered with down, and 

 have open eyes, can maintain reasonably 

 homeothermic body temperature, and leave 

 the nest site to feed themselves after a few 

 hours or days. At hatching, semiprecocial 

 chicks appear similar to precocial chicks, al- 

 though they are slightly less well developed 

 (Ricklefs 1974; Dunn 1975a). In contrast to 

 precocial chicks, they remain at the nest site 

 for some time, are fed by their parents, and 

 tend to grow rather rapidly (Ricklefs 1968). 

 Altricial nestlings hatch at a much less ad- 

 vanced stage of development. They are naked, 

 blind, helpless, essentially poikilothermic, and 

 depend completely on their parents for food 

 and shelter. They usually remain at the nest 

 until full grown. Semialtricial chicks show 

 somewhat intermediate characteristics (Nice 

 1962). 



The amount of yolk (and therefore energy) 

 in an egg is positively correlated to the degree 

 of development at hatching (Table 3). The 

 same is true for egg size: altricial and semi- 

 altricial birds have smaller eggs relative to 



Table 3. Amount of yolk in eggs of different 

 types of birds. After Ricklefs (1974). 



adult body weight than do semiprecocial and 

 precocial birds (Fig. 8). Clutch size, however, 

 is unrelated to energy content of the eggs. For 

 example, shags (which are altricial) and eiders 

 (precocial) have among the largest clutches of 

 northern seabirds (four to six eggs). 



EGG WEIGHT - G 



m o 

 o 



X 

 H 

 I _ 



o o 

 o 

 o 



Fig. 8. Egg weight as a function of body weight in 

 various northern seabirds. Solid symbols repre- 

 sent precocial and semiprecocial species, and 

 open symbols altricial and semialtricial species: 

 solid circles, alcids; solid triangles, gulls, terns, 

 and jaegers; solid squares, eiders; open squares, 

 cormorants and Moms bassanus; and open cir- 

 cles, petrels. Data from Belopol'skii (1961); Drent 

 (1965); Schonwetter (1967); Lack (1968); B6dard 

 (1969a); Cody (1973); Sealy (19736); Harris (1974); 

 and Manuwal (1974a). 



The energetic cost of egg laying depends 

 not only on caloric content of the egg and 

 clutch size, but also on speed of development. 

 Ricklefs (1974) has shown that the energetic 

 cost per day of egg laying can be calculated 

 from the energy content of the yolk and white, 

 clutch size, the amount of follicular growth 

 per day, and the laying interval between eggs. 

 The energy content of a single egg (expressed 

 as percentage of BMR) has been estimated as 

 follows: 45 (altricial passerines), 103 (semial- 



