Dispersal
In general terms, the willingness of some individuals to disperse while the majority of individuals remain loyal to a colony can be considered a major mechanism of population maintenance. If conditions deteriorate seriously at one place so that the local populations decline or disappear, dispersal from other centers can be expected to repopulate the area as soon as local conditions again become suitable. This subject has been treated in more detail by Drury and Nisbet (1972) and Drury (1974b).
Occupation of new, or return to former, nesting sites has been recorded in detail for fulmars (Fulmaris glacialis) by Fisher (1952) and for herring gulls by Kadlec and Drury (1968). Dispersal is also known for waterfowl. Hansen and Nelson (1957) reported that of some 8,000 brant banded in midsummer on the Yukon delta 8 were recovered in northern Siberia and 28 in northern Alaska and arctic Canada. They suspected that pairing on the wintering grounds was responsible for the change in breeding areas, a change that would not be expected among other North American species of geese. Similarly, wide dispersal seems to occur in pintails (Anas acuta), mallards (Anas platyrhynchos), and wood ducks (Aix sponsa).
The general tendency for some individuals to disperse and the frequency of "extra limital" breeding attempts is especially well established in the Bering Sea region, in part at least because vagrants from Siberia or North America are readily identified as such. In the Aleutian Islands, Emison et al. (1971) and Byrd et al. (1974) have enumerated the nesting vagrants. For the Pribilof Islands, Kenyon and Phillips (1965), Sladen (1966), and Thompson and DeLong (1969) have recorded the repeated appearance of birds of Siberian distribution, and Fay and Cade (1959) and Sealy et al. (1971) did the same for St. Lawrence Island.
One can conclude that a few individuals are constantly trying to settle in new geographical areas. As climatic and habitat conditions change, some populations are able to become established; for example, southern species such as mockingbirds (Mimus polyglottus), cardinals (Cardinalis cardinalis), and tufted titmice (Parus bicolor) have settled in southeastern New England during the last 2 decades. These southern species have received much publicity. But at the same time, a less publicized dispersal of white-throated sparrows (Zonotrichia albicollis), hermit thrushes (Catharus guttatus), and dark-eyed juncos (Junco hyemalis) has resulted in new nesting records of more northerly species, also in southeastern New England.
The ability (or lack of ability) of some organisms to expand their ranges over time has been a subject of consideration for a number of years by plant and animal geographers. An important botanical paper on this subject in the Bering Sea region was presented by Hultén (1937), who analyzed the ranges of plants of the area of Kamchatka, eastern Siberia, Alaska, and northwest Canada, showing that diverse floras occur in some restricted geographic areas. He called these areas "refugia," and postulated that many species had survived Pleistocene glaciations in them because these refugia remained ice-free. He, like Fernald (1925), was puzzled as to why only certain species had been able to expand their ranges outward from these "areas of persistence," while other apparently more "conservative" species were unable to do so. Similarly, there appear to be conservative endemic bird species of the Bering Sea region: the extinct Commander Islands cormorant (Phalacrocorax perspicillatus), Steller's eider (Polysticta stelleri), spectacled eider (Lampronetta fisheri), emperor goose (Philacte canagica), whiskered auklet (Aethia pygmaea), least auklet (A. pusilla), parakeet auklet (Cyclorrhynchus psittacula), Aleutian tern (Sterna aleutica), red-legged kittiwake (Rissa brevirostris), bristle-thighed curlew (Numenius tahitiensis), long-billed dowitcher (Limnodromus scolopaceus), surfbird (Aphriza virgata), black turnstone (Arenaria melanocephala), rock sandpiper (Calidris ptilocnemis), and western sandpiper (C. mauri).
The ranges of horned puffins (Fratercula corniculata), Kittlitz's murrelet (Brachyramphus brevirostris) and, perhaps, crested auklet (Aethia cristatella) suggest that some species of "Beringian" seabirds have expanded their ranges from Hultén's (1937) "refugia."
Dispersal and Regional Persistence of Marginal Populations
The presence of several sub-elements of a species population and, therefore, the opportunity for dispersion among alternative breeding sites may be an important factor in the regional persistence of a species on the margin of its range, as illustrated by the history of laughing gulls (Larus atricilla) in New England.
Between 1875 and 1900 there were fewer than 50 laughing gulls in Massachusetts (Mackay 1893) and about 35 in Maine (Norton 1924). In Massachusetts the laughing gulls all settled on one large island, Muskeget, where by 1940 there were about 20,000 pairs (Noble and Würm 1943). Meanwhile, in Maine the population had been disturbed by sheep and men and had shifted about among seven islands. The Maine population grew to only about 350 pairs by 1940 (Palmer 1949).