Fig. 8. Egg weight as a function of body weight in various northern seabirds. Solid symbols represent 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 Morus bassanus; and open circles, petrels. Data from Belopol'skii (1961); Drent (1965); Schönwetter (1967); Lack (1968); Bédard (1969a); Cody (1973); Sealy (1973b); 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 (semialtricial raptors), 126 (precocial galliformes), 180 (precocial ducks), 226 (precocial shorebirds), 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 (expressed 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 required for calculation of the average energetic cost of a clutch are not available for other northern seabirds.
For no species have all the additional factors 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 re-nesting 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 entirely by the female, although males may contribute some time and energy toward egg laying 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 laying can be profoundly influenced by the degree 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 predation. As is shown in Table 4, the degree of 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 laying by a given species or population. Genuine second clutches are occasionally laid by phalaropes (Hilden and Vuolanto 1972) and Cassin's auklets (Manuwal 1974a).
| Nest exposure | Nests destroyed by predators (%) |
|---|---|
| Completely hidden | 3.2 |
| Partly exposed | 5.8 |
| Largely exposed | 13.6 |
| Completely exposed | 18.2 |
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 factors, such as age, sex, degree of nest destruction, weather, other species present, and feeding 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 affecting growth rate (discussed later) apparently affect incubation period as well.