Comparisons Between Hemispheres

Migration of landbirds from their breeding areas is a much more obvious phenomenon in the northern hemisphere than in the southern. This is partly because land covers three times the area in the northern hemisphere as in the southern hemisphere, and the difference is most marked at high latitudes (we can ignore Antarctica because it holds no landbirds) (Figure 13.6). In North America, Greenland and Eurasia, some landbird habitat extends north of 80°N, but in the southern hemisphere, South America reaches only to 55°S, Africa to 35°S, Australia to 43°S, and New Zealand to 47°S. The net result is that latitudes 30-80°N hold 15 times more land than do latitudes 30-80°S, and it is at these latitudes that winters are coldest, and migration is most developed (for discussion of area effects in South America see Chesser 1994, and in Australia see Chan 2001). The greater latitudinal spread of land in the northern hemisphere results not only in more marked migration, but also in generally longer journeys than are undertaken by southern hemisphere breeders, which are closer to the equator. These factors are likely to explain the relative proximity of breeding and wintering ranges typical of many southern hemisphere migrants (Chesser 1994, Jahn et al. 2004).

(a) Index of ice-free land area

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