Evidence from ecological niches

Most species seem to occupy ecological niches in their wintering areas that are similar to those they use in their breeding areas. They occupy places with similar climate at the time they are there, habitats of similar structure, and take similar types of foods in similar ways. The actual plant species that comprise the habitat, or the plant or animal species that comprise the diet, may differ between breeding and wintering areas, but superficially they look much the same. However, some bird species make switches in climate types or habitats between seasons, such as inland marsh to open ocean, or forest to scrub, or they make switches in the types of foods eaten, such as insects to seeds. Extreme examples include the sandpipers that switch from tundra to seashore and from insects to worms and molluscs, or the skuas that switch from tundra to the open sea and from lemmings to fish. Hypotheses regarding the evolution of migration would generally predict 'niche-following' as primitive, and 'niche-switching' as derived, and thus as representing a further step in the evolution of migratory from resident populations.

Examination of 21 species of New World migrants (nine Dendroica, four Spizella, three Vermivora and five Vireo species) revealed that the majority occupied a particular climatic regime year-round, seeking areas of similar climate in summer and winter, while others make a clear switch in climatic regime (and presumably also in other aspects of their ecology) between seasons (Nakazawa et al. 2004). In the 'niche-switchers', the winter habitat seemed ancestral and the summer habitat derived. Niche-switching is unavoidable for many of the small insectivores that breed in boreal forest, because no coniferous habitat is available at the low tropical latitudes where they winter, and in which many probably evolved. Niche constancy in terms of climate had earlier been noted in Swainson's Flycatcher Myiarchus swainsoni (Joseph & Stockwell 2000).

A further study involving the directionality of evolution of gross habitat preferences within the Passerina buntings again indicated greater evolutionary changes in the niches of breeding populations than of wintering populations (Martinez-Meyer et al. 2004). These results were consistent with the hypotheses of: (1) niche conservatism (in winter at least) across a recently speciated lineage; and (2) the derived state of the breeding (rather than wintering) niches of each species - a further indication that, in range expansion, many birds have moved to new breeding areas rather than to new wintering areas.

Uncertainty surrounds species that occupy habitats in the non-breeding season in which they could not breed, such as shorebirds which nest on the arctic tundra and winter on lower latitude intertidal mud, or skuas which nest on the tundra and winter in lower latitude sea areas. During glacial periods, the breeding and wintering areas of such species may have been less separated than today, but if these birds always used the same habitats as they do now, they must always have migrated to lower latitudes for the winter when arctic shorelines and seas freeze over. This might seem like a clear case of birds developing migration from breeding to wintering areas. However, if such species originally nested in other open habitats (such as saltmarsh) near seas that did not freeze in winter, they could have fitted the same pattern as proposed for other species, spreading to higher latitudes and developing migrations as they reached areas where wintering was not possible. Short-distance change in habitats between breeding and non-breeding seasons is known now in many shorebirds that winter in intertidal areas and nest on nearby shingle (Ringed Plover Charadrius hiaticula, Oyster-catcher Haematopus ostralegus) or saltmarsh (Redshank Tringa totanus, Dunlin Calidris alpina). In these respects, such populations are little different from the many pelagic seabirds which feed from the sea and nest on nearby coasts and islands. With many shore-birds, migrations probably developed so far back in time that it is impossible to tell from current patterns how they evolved.

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