of year. The same is true for other regions. Evidently, the migrations of most species hinge around the annual temperature cycle rather than around the daylength cycle. It is the temperature cycle that, at high latitudes, has most influence on vegetation growth and bird food supplies.

Within any one breeding area, the timings of spring arrival and autumn departure differ between species according to when their particular food supplies re-appear in spring and collapse in autumn. In general, regardless of latitude, those species that arrive relatively early in their breeding areas depart relatively late, but exceptions occur (Figure 14.2). Moreover, the peak migration dates for different species at particular localities are spread over a shorter period in spring than in autumn (Figure 14.1), and within most (but not all) species spring arrival occurs over a shorter period than autumn departure (Chapter 12).

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