The far-reaching works of man in altering the natural conditions of the earth's surface can so change the environment necessary for the well being of the birds as to bring about changes in their yearly travels. (Frederick C. Lincoln 1935.)
We saw in the previous chapter how the migratory habits of birds can be rapidly altered under the influence of selection, continually shaped and re-shaped in response to changing conditions. Change in the migratory behaviour of wild birds has attracted attention in recent years as a result of growing interest in the effects of climate change. If weather has become warmer, as it has over much of the world, one might expect birds to have responded accordingly, with migratory species wintering at higher latitudes than previously, or arriving earlier and departing later from their breeding areas. This gives special interest in the measurement of changes in migratory behaviour, derived from long-term observations or ring recoveries.
A second aspect of interest concerns the basis of any changes observed. Many have assumed that recorded changes in migratory behaviour are likely to be genetically controlled, providing examples of 'evolution in action'. Genetically-based changes are not hard to imagine. Consider a partially migratory species in which some individuals have a genetic propensity to migrate and others not. During a series of severe winters, the migrants would survive better than the residents. As a result, the offspring of the migratory genotypes would comprise an ever greater proportion of each succeeding generation, gradually changing the genetic composition and average migratory habits of the population. Conversely, during a series of mild winters, the resident genotypes, able to occupy the best territories and start nesting early, could come rapidly to outbreed and outnumber the migrants. However, many of the changes observed in bird migratory behaviour need entail no genetic change, for in every aspect of migration there is scope for individual flexibility, through which individuals can adjust their migratory behaviour to some extent according to prevailing conditions (facultative variation). For example, the same birds might arrive on their breeding areas earlier in warm springs than in cold ones, or they might migrate further in cold winters than in mild ones, in response to differing food supplies. Hence, as climate changes from year to year, or over longer periods, birds have considerable scope for adjusting their behaviour to match these changes, without the need for any modification in the genetic control mechanisms.
Many of the changes in migratory behaviour witnessed in recent decades, which parallel changes in climate or food supplies, could therefore be facultative in nature, lying within the pre-existing range of response behaviour, and requiring no genetic change. Without special study, on a case-by-case basis, there is no way of telling to what extent any of the observed changes in migratory behaviour have resulted from the action of natural selection on the gene pool of the population. For the most part, we can only measure the extent of changes, and their association with environmental change. The following sections illustrate the different types of change observed in bird populations in recent decades, with various examples given in Table 21.1.
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