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In particular regions, seasonal changes in the food supplies of birds are driven by seasonal changes in daylength and weather - mainly by temperature at higher latitudes and by rainfall at lower latitudes. These seasonal changes ultimately influence how much of each year migrants can remain in their breeding areas without jeopardising their survival prospects. With increasing latitude, spring begins later and autumn begins earlier, shortening the growing season for plants, the activity season for insects, and the potential breeding season for birds.

Away from the equator, the annual temperature cycle typically lags about one month behind the daylength cycle. In the northern hemisphere, while the longest day falls on 21 June, the warmest day in any particular locality falls, on average, around 21 July. Similarly, while the shortest day occurs on 21 December, the coldest day falls, on average, around 21 January (Preston 1966). The peak dates of spring and autumn migration for various bird species at different localities in North America are shown in Figure 14.1. At each locality, the mid date between spring and autumn migration dates averages around 17 July, the warmest time

Figure 14.2 Relationship between median spring and autumn migration dates for various species in (a) Alaska (64° 50'N, N = 18);

(c) Pennsylvania (40° 40'N, N = 33). In general, species that arrived early tended also to depart late, and vice versa. Regression relationships: Alaska, b = 1.09, r = 0.69, P < 0.002; Minnesota, b = 0.61, r = 0.43, P < 0.01; Pennsylvania, b = 0.97, r = 0.78, P < 0.001. Details from Preston (1966), Winker et al. (1992a), Benson & Winker (2001).

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