Among northwestern North American seabirds, most coastal feeders, such as gulls, cormorants, and many alcids and petrels, have only a short southward migratory movement, and many others are more or less resident (Dorst 1961; Ashmole 1971). Terns, on the other hand, migrate long distances in a short time to places where small fish are available near shore in the winter. Other long-distance migrants—Sabine's gull (Xema sabini), jaegers (Stercorarius spp.), pelagic phalaropes (Phalaropus, Lobipes), and kittiwakes (Rissa spp.)—tend to scatter widely over the southern ocean, concentrating near areas of upwelling (Dorst 1961; Ashmole 1971). Groups such as murres (Uria spp.), eiders (Somateria spp.), and grebes (Podiceps spp.) may move considerable distances by swimming (Dorst 1961; Tuck 1960). True migration tends to take place directly before and after reproduction, whereas dispersal or nomadism takes place over a long period of the winter (Fig. 4).
Among species remaining in the northern hemisphere, younger birds frequently disperse greater distances than do breeding adults (Coulson 1961; Kadlec and Drury 1968; Southern 1967), and the degree of dispersal can vary among colonies of the same species (Coulson and Brazendale 1968).
Energy costs of migration must vary according to distances covered and amount of time allocated to migration. Aside from the references to cost of flight mentioned earlier, however, migratory costs have scarcely been studied. Dolnik (1971) has estimated that chaffinches (Fringilla coelebs) expend about as much energy migrating south as they would on thermoregulation if they overwintered on their breeding grounds. Long-distance migration is presumably selected because the birds are able to collect food more efficiently, because the risks of death or injury in migrating are less than in residency, and so on. Interspecific and intraspecific competition may also be involved (Cox 1968). In other words, migratory patterns are selected to optimize survival and reproduction in alternating environments (Cohen 1967; Drury and Nisbet 1972).
| Rapidity of wing molt and (indented) flight capability in molt | Timing of start of molt | Species |
|---|---|---|
| Slow Retained | During care of young | Cassin's auklet, parakeet auklet (Cyclorhynchus psittacula), whiskered auklet (Aethia pygmaea) |
| Retained | After young become independent | Least auklet (A. pusilla), crested auklet (A. cristatella) |
| Rapid Poor | After arrival in winter quarters | Marbled murrelet, Kittlitz's murrelet (Brachyramphus brevirostris) |
| Almost synchronous None | After end of breeding | Xantus' murrelet (Endomychura hypoleuca) |
| Synchronous None | As soon as young go to sea | Guillemots (Cepphus spp.), murres, razorbill (Alca torda), dovekie (Alle alle) |
| None | In winter, after body molt | Puffins |
Although one may suspect that location of winter food supply is the main environmental factor affecting migratory patterns, there is little direct evidence on the reasons for, or the benefits accruing from, the different patterns seen in seabirds. Study of cost-benefit ratios of foraging in different stages of migration might help clarify the question.
Molt
Patterns of molt vary widely among seabirds. The commonest pattern is for a prenuptial body molt to occur in spring, and for an extended wing molt to begin after the breeding season and continue well into the winter (Fig. 4). In short-distance migrants, molt may overlap slightly with the end of breeding and can last up to 6 months, as in most gulls, terns, alcids, nonmigratory jaegers, and cormorants (Stresemann and Stresemann 1966).
Long-distance migrants frequently delay molt until in the winter quarters (lesser black-backed gull, Larus fuscus; Sabine's gull; jaegers; arctic tern, Sterna paradisaea; and marbled murrelet, Brachyramphus marmoratus) and molt there may occur rapidly (3.5 months in the arctic tern). Certain other long-distance migrants begin molt before leaving the breeding grounds (herring gull; skua, Catharacta skua; Leach's petrel, Oceanodroma leucorhoa; and fulmar), although molt may be interrupted during migration, as in Larus argentatus heuglini (Stresemann and Stresemann 1966). Duration, timing, and rapidity of molt are particularly varied among the alcids (Table 2).
A few unusual molt patterns are found in northern seabirds. The ivory gull (Pagophila eburnea) has its major annual wing and body molt immediately before it breeds. In several other species such as the glaucous gull (Larus hyperboreus) and Cassin's auklet (Ptychoramphus aleuticus) the molt almost completely overlaps the reproductive cycle (Johnston 1961; Payne 1965). Potts (1971) documented a molt pattern in shags (Phalacrocorax aristotelis) which is more typical of tropical seabirds. Several cycles of wing molt take place simultaneously, each lasting more than a year, and molt ceases in winter. By the time breeding age is reached, each flight feather is replaced once a year.