Some birds seem to respond by movements to predation and disturbance (Chapter 27). For example, the banning of hunting in two coastal areas of Denmark resulted in waterfowl staying in larger numbers than in previous years, delaying their onward migration south (Madsen 1995). Over a five-year period, these reserves became important staging areas, where most quarry species stayed up to several months longer each winter than in earlier years. In contrast, no changes in waterfowl numbers were noted in other areas of Denmark still open to wildfowling, so the accumulation of birds in the reserves was attributed to individuals remaining there, rather than moving further down the migration route. Birds presumably respond in the same way to natural predators which, like human hunters, could kill some individuals and continually disturb others, preventing them from getting enough food (Chapter 27).
The important point to emerge from the above sections is that migration is not always a simple two-season movement between fixed breeding and wintering areas. Some species move between three distinct areas: a breeding area, a late summer or autumn area, and a wintering area. Others move only when necessary, and no further than necessary, wintering much nearer to their breeding areas in some years than in others, and thereby saving on the costs of migration. Yet others seem to spend much of their lives on the move, following the same route each year, pausing for a few weeks here and a few weeks there, before moving on. Their mobile lifestyle enables them to exploit short-lived food supplies at different places at different times, as they occur (Newton 1979, Lack 1990, Pearson & Lack 1992, Jones 1995). It is a lifestyle that birds more than most other kinds of animals can adopt to the full. These various examples of movements within the breeding or non-breeding seasons provide further circumstantial evidence for the underlying role of food supplies in governing the migration patterns of birds.
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