Within these broad categories of molt pattern there are sometimes variations according to age, sex, and even subspecies (Stresemann and Stresemann 1966). Male common eiders (Somateria mollissima) molt directly after mating, when their reproductive role is completed, whereas females molt only after they have taken their young to sea. Nonbreeders and failed breeders frequently begin molt while other adults are still raising young and not molting—e.g., many alcids, gulls, storm-petrels, and fulmars (Stresemann and Stresemann 1966; Ingolfsson 1970; Harris 1971; Harris 1974; Sealy 1975b). In ivory gulls, which molt just before reproduction, and in Sabine's gulls, which complete molt just before breeding, nonbreeders may extend wing feather growth into the breeding season (Stresemann and Stresemann 1966).
There is little information on the energetic cost of molt, although there are indications of at least some expense. Belopol'skii (1961) showed that nonmolting seabird species tended to gain weight after reproduction, whereas those that immediately started molt tended to lose weight. Among other birds, however, it is common for individuals to gain weight just before, and even during molt (Payne 1972). The BMR is known to rise in molting birds (Blackmore 1969; Lustick 1970; Payne 1972), from as little as 5% to as much as 34% above nonmolting levels. In one study, about 35-40% of the increased BMR represented extra thermoregulatory costs incurred by lessening of insulation and increase in heat loss from well-vascularized new feathers; the rest of the increase represented the energetic cost of growing feathers (Gavrilov 1974). The fact that molt rarely overlaps with breeding suggests that the energetic cost, even if slight, may be incompatible with the already high costs of reproduction (Payne 1972). Cassin's auklets, which do molt while breeding, may cease molt while feeding large young (Payne 1965), and certain species interrupt molt during migration (Stresemann and Stresemann 1966). Doubtless a rapid simultaneous molt is more costly than a long gradual one.
Rapid molt appears to occur at a time in the annual cycle when food resources are abundant (spring or late summer), whereas extended molt generally occurs over winter (e.g., Bédard 1969a). If one speculates that energy availability is the main determinant of molt patterns, one can also speculate on the cause behind some of the more unusual patterns. Possibly birds in which molt and breeding overlap either have extraordinary available energy at that time or else face shortages in other periods. For example, ivory gulls, which breed in the high Arctic, molt when food resources have become abundant in the low Arctic but before the high Arctic breeding grounds have thawed sufficiently for reoccupation.
Speed of molt may also reflect availability of energy resources or of nutrients needed for feather growth (Payne 1972), but must also be influenced by the need for full flight capabilities to obtain food. The eider duck and many alcids that shed wing feathers almost simultaneously do not need their wings for flight after the young have left the breeding colony. Hydrodynamic considerations suggest that their fishing capabilities may even be improved (Storer 1960). This is not true for the smaller species—e.g., Aethia molts only one feather at a time and retains full flight capabilities (Table 2). Climate may also influence simultaneity of molt if heat loss in rapid molt is particularly severe.
Reproduction
Time use of seabirds is best known for the reproductive period, when the birds are on relatively accessible breeding grounds, the weather is most suitable for observation, and academic researchers are freed from their jobs. Even so, the details of timing are known for only a few of the species and localities on the northwest North American coast (e.g., Drent and Guiguet 1961; Drent et al. 1964; Cody 1973; Sealy 1973a, 1975b, 1975c). The following discussion emphasizes the multitude of environmental factors known to influence timing and total length of time devoted to various aspects of the reproductive cycle.
Timing of the Season
Each species of seabird returns to the colony site when weather conditions have ameliorated sufficiently to meet its particular needs. For example, the early arrivals to islands in the Barents Sea are murres, kittiwakes, and herring gulls, which need only small cracks in the sea ice to meet their feeding requirements (Belopol'skii 1961). Eiders in North America also return early, when a few ice leads have formed (Schamel 1974). Common puffins (Fratercula arctica) and mew gulls (Larus canus) are somewhat later arrivals, and terns and a few parasitic jaegers (Stercorarius parasiticus) are the latecomers to Barents Sea colonies (Belopol'skii 1961).
The timing of the season (as illustrated in Fig. 5) varies widely among localities, and because of local weather patterns and ocean currents, this variation can be unrelated to latitude (Belopol'skii 1961). Examples of such variation are also known in North America: for instance, Leach's storm-petrels in Alaska lay eggs 2 to 3 weeks later than do those in California (Harris 1974); however, the details of timing are largely unknown for many species in this region. Progression of thaw, which also varies from year to year, causes variation in the timing of the breeding season (Belopol'skii 1961; Evans and McNicholl 1972). Fig. 6 shows the diversity in start of the breeding season for different species on the same island in the Barents Sea as well as variation in time devoted to various components of the reproductive cycle.
