Timing of egg laying is influenced not only by weather (Erskine 1972; Sealy 1975c), but also by numerous biotic factors. Smith (1966) showed that where glaucous gulls, herring gulls, and Thayer's gulls (Larus thayeri) breed in mixed colonies, the peak of sexual activity and egg laying in Thayer's gull is about midway between the peaks for the other two species (Fig. 7). In nearby colonies where herring gulls are absent, however, the peak of sexual activity in Thayer's gulls is delayed about a week, and activity continues for a significantly longer period (Fig. 7).
Fig. 7. Timing of peak sexual activity (a combined measure of egg laying and testes size) in colonies of arctic gulls of different species composition. From Smith (1966).
Annual variations in food supply also will affect the start of the egg-laying season. Belopol'skii (1961) cited an example from the Barents Sea in 1940 when a series of storms made it difficult for certain seabirds to find food. Murres and kittiwakes, which were able to catch fish, started reproductive activities on schedule. Gull breeding was delayed, however, and egg laying began in force only after fishing boats arrived and started discarding offal. Onset of egg laying in great cormorants (Phalacrocorax carbo) is correlated to April air temperatures (Erskine 1972), and this may also be related to variations in spring increase of food availability. In certain birds the breeding season has been shown to start particularly early when food supplies are unusually abundant (Högstedt 1974; Källender 1974), but this has not yet been demonstrated in seabirds.
Lastly, age and sex of seabirds are known to affect the timing of egg laying (e.g., Coulson and White 1960; Lack 1966); older, more experienced birds tend to lay earlier than do younger ones. In shags, males tend to breed progressively earlier as they increase in age, but females do not (Snow 1963).
Contrary to the situation in passerines, seabirds 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). Precocial chicks are hatched at a relatively advanced 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, although 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 advanced 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).
| Developmental type | Percent yolk (by weight) |
|---|---|
| Precocial | 30-60 |
| Semiprecocial | 25-30 |
| Altricial | 15-25 |
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 semialtricial birds have smaller eggs relative to 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).