Once an inherited tendency to move in a particular direction outside the breeding season is established, the distance travelled may increase over successive generations if the individuals that move furthest happen to have the highest survival.
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As a result, there may be a noticeable seasonal shift in the range occupied by the population as a whole, because for a proportion individuals, movement occurs to a region that is outside of the area that provides all the necessary requirements of breeding.
If the underlying gradient of environmental suitablility is sufficiently extensive, the breeding and non-breeding area may ultimately become separate.
Another consequence of the development of migratory behaviour is that individuals can breed in locations where survival is too low to sustain a resident population. Gradients in food availability are often seasonally reversed: e.g. in temperate areas food abundance declines at higher latitudes in winter, but increases in summer.
Migrants that disperse into areas unsuitable for residents can therefore take advantage of food abundance during the high season, and escape scarcity because of their obligatory migration in the direction of greater low season abundance, and so establish a viable breeding population. |
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