152 



E. H. DUNN 



tricial raptors), 126 (precocial galliformes), 

 180 (precocial ducks), 226 (precocial shore- 

 birds), and 320 (semiprecocial gulls and terns). 

 Gulls and terns thus have very costly eggs, as 

 well as a moderately high clutch size (three). 

 However, the development time for a single 

 ovum in the herring gull is unusually long 9 

 to 10 days (King 1973). Ricklefs (1974), who 

 calculated the energetic cost per day (ex- 

 pressed as percentage BMR), estimated the 

 cost of a clutch in gulls and terns (120% BMR 

 per day) to be similar to that for various 

 groups of precocial birds (about 125-180% 

 BMR per day). Unfortunately, the data re- 

 quired for calculation of the average energetic 

 cost of a clutch are not available for other 

 northern seabirds. 



For no species have all the additional fac- 

 tors influencing the energetic cost of a clutch 

 been taken into account. For example, eggs in 

 a clutch may vary in size (and caloric content) 

 according to sequence in the clutch (Preston 

 and Preston 1953; Snow 1960; Coulson 1963; 

 Coulson et al. 1969). Age has a definite effect 

 on laying energetics, as older birds lay larger 

 eggs (Coulson and White 1958; Snow 1960; 

 Coulson 1963; Coulson et al. 1969) and lay 

 larger clutches (Coulson and White 1960; 

 Snow 1960). They also lay, on average, earlier 

 in the season (Coulson and White 1958; Snow 

 1960; Coulson et al. 1969), and eggs laid late 

 in the season (whether by young birds or older 

 ones in renesting attempts) tend to be smaller 

 and contain less energy. In addition, egg 

 quality can vary with food supply: Snow 

 (1960) found eggs to have more yolk in years 

 when food was abundant than in years when 

 food was scarce. 



Egg-laying costs are, of course, borne en- 

 tirely by the female, although males may con- 

 tribute some time and energy toward egg lay- 

 ing through courtship feeding (Ashmole 1971; 

 Henderson 1972; Nisbet 1973). Courtship 

 feeding takes place in most lariforms but not 

 in eiders, phalaropes, or cormorants. 



The time and energy expended on egg lay- 

 ing can be profoundly influenced by the de- 

 gree of nest destruction, since females usually 

 lay a replacement clutch if the loss of the first 

 does not occur too late in the season. Factors 

 causing egg destruction are numerous, but 

 among the most important in the north is pre- 

 dation. As is shown in Table 4, the degree of 



Table 4. Predation on nests of common murres 

 according to degree of exposure. From 

 Belopol'skii (1961). 



Nest exposure 



Nests destroyed 

 by predators 



Completely hidden 

 Partly exposed 

 Largely exposed 

 Completely exposed 



3.2 



5.8 



13.6 



18.2 



egg predation in common murres is correlated 

 to degree of exposure of the nest so even 

 such an unlikely sounding factor as physical 

 characteristics of the nest site can affect the 

 average time and energy expended on egg lay- 

 ing by a given species or population. Genuine 

 second clutches are occasionally laid by phala- 

 ropes (Hilden and Vuolanto 1972) and 

 Cassin's auklets (Manuwal 1974a). 



In short, time and energy devoted to egg 

 laying depend not only on the species, but also 

 on a multitude of other biotic and abiotic fac- 

 tors, such as age, sex, degree of nest destruc- 

 tion, weather, other species present, and feed- 

 ing conditions. 



Incubation 



The total time devoted to incubation does 

 not depend directly on developmental type or 

 egg size but differs markedly among families 

 (Lack 1968). Since incubation period seems to 

 be closely linked to fledging period, factors af- 

 fecting growth rate (discussed later) ap- 

 parently affect incubation period as well. 



Each species has a different incubation 

 rhythm. In birds in which the sexes share in 

 incubation, the sexes exchange places at inter- 

 vals 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 (Manu- 

 wal 1974a; Sealy 1975a); 33 h (on the average) 

 in common puffins (Myrberget 1962); 72 h in 

 ancient murrelets, Synthliboramphus an- 

 tiquus (Sealy 1975a); and 96 h in Leach's 

 storm-petrel (Gross 1935). Degree of atten- 

 tiveness once a bird is on the nest also varies. 

 Petrels may leave the egg for several days 



