The amount of time and energy spent by the male and female in nest building differs among species. In Leach's storm-petrel, the male digs the burrow (Gross 1935), whereas in eiders, the nest is built entirely by the female. In most seabird species, the sexes share in nest construction, but roles may still be separated. For example, in shags the male collects the nest material and the female builds the nest (Snow 1963).
Timing of egg laying is influenced not only by weather (Erskine 1972; Sealy 1975_c_), 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).
[Illustration: 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).