In this chapter, I have been concerned mainly with changes in the numbers of migratory birds, and with the relative importance of events in breeding and wintering areas in influencing these changes. In the longer term, however, population limitation in migrants is probably a dynamic process, involving both areas. If conditions in the wintering range permit increased survival, a species could become more numerous, and may expand its breeding distribution, perhaps into places where the production of young is lower. Similarly, if conditions in the breeding range permit increased production of young, the species could become numerous enough to expand its wintering range, perhaps into areas where survival is lower. In this way, the summer and winter ranges of migrants will tend to expand until reproduction and mortality balance (see also Cox 1985). It is only during a period of change, as in recent decades, that the breeding or wintering range may emerge as providing the stronger limitation. These speculations assume that there are indeed vacant areas into which migrants could expand if their numbers rose. For most species this is probably true; either other habitats within the existing range or other suitable areas outside it.
Migratory birds also depend on encountering suitable conditions at staging points on their routes (Chapter 27). If conditions deteriorate at any one point, a food bottleneck might develop that could begin to limit the population. When conditions are deteriorating everywhere at once, it becomes hard to pinpoint that bottleneck except in the most obvious cases. But the fact that migrants use two or more essential areas each year means that they are inevitably more susceptible to the effects of habitat destruction than are resident birds. Residents suffer only if their particular area is destroyed, but migrants could suffer if any one of several areas important to them were lost. In this sense they have, on the average, more chances than residents of being affected - adversely or otherwise - by human action.
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