Bird Migration and Natural Selection

Abstract

With the present interest in analysing and experimenting upon migration, it is important to keep in mind that it is a product of natural selection, but the danger of the evolutionary approach is that it is speculative. 2. It is probably advantageous for migrant birds to leave their breeding grounds because they could not find enough food there in winter (though they usually leave before food is short), and to return there because they can find more food for their young there than in crowded winter quarters. 3. Correspondingly, the size of the world population of a migrant species is probably determined by winter supplies and its breeding dispersion (range, habitat etc.) by where it can find most food for its young. 4. The critical factor is not the absolute quantity of food, but the amount available in relation to actual or potential competition. In- direct evidence for this is provided by the existence of competitive exclusion among related species on the wintering grounds. 5. The advantages of, respectively, migration and winter residence are roughly equal in partial migrants, but they differ in different places, different years in the same place, and different sexes and age-groups. 6. The historical factor in migration was formerly over-stressed, but it has had an in- fluence on directions taken because it is hard to evolve a successful hereditary change in direction. 7. Movements into or out of England occur almost every day throughout the winter, hard- weather movements south or west because food becomes scarce, return movements in each mild spell presumably owing to intense competition in the crowded milder areas. 8. Irruptive movements, like migration, can have been evolved through natural selection. Pe3loMe