Records indicate that many bird species have changed some aspects of their migratory behaviour during the last century or more, in response to changed conditions, with (1) earlier arrival in spring, (2) earlier or later departure in autumn, (3) shortening or lengthening of migratory routes, (4) directional changes, and (5) reduced or enhanced migratoriness, reflected in changes in ratios of resident to migratory individuals in particular breeding areas, and in the occurrence of wintering birds in regions previously lacking them. Almost all these changes are associated with changes in food availability, or with climatic conditions that are likely to affect food supplies, such as milder winters. Most examples of shifts to increasing migratoriness involve species that have extended their breeding ranges into higher latitude areas where overwintering is not possible or risky.
Some of the observed changes in migratory behaviour could represent an immediate (facultative) response to prevailing conditions, and others may have resulted from genetic changes brought about through the action of natural selection. Despite the difficulties of detecting genetic changes, evidence for a few species has indicated genetic changes in migration timing and, for at least one species, in migratory intensity and direction. In practice, most changes in migratory behaviour are likely to start as facultative responses which become genetically entrenched as selection comes to act in a consistent manner from year to year.
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