the most extreme example, breeding between about 50° and 80°N and wintering between about 40° and 70°S, giving a 90° latitudinal gap between the breeding and wintering ranges.

Comparison of sizes of breeding and wintering areas

Whether overlapping or separate, those migrant species that have the largest breeding ranges also tend to have the largest wintering ranges, and vice versa (Newton 1995a). This point is illustrated in Figure 13.9 for 57 species of landbirds that breed entirely within Eurasia and winter entirely within Africa, so that their breeding and wintering ranges are completely separated. As another reflection of the same phenomenon, European landbirds and freshwater birds which breed over the widest span of latitude also winter over the widest span of latitude, and vice versa (Figure 13.10). These correlations may have their basis in the ecology of the species themselves, in that those species that have the widest climatic and habitat tolerances may be able to spread over the largest areas, summer and winter. Alternatively, the correlations may depend on the abundance of the species concerned, in that those that have the largest populations (for whatever reason) spread over the largest areas, summer and winter (Newton 1995a). These two explanations are not mutually exclusive, and in practice are difficult to separate.

The correlations between sizes (or latitudinal spans) of breeding and wintering areas hold only as general tendencies, however, and some species do not fit the overall patterns. Moreover, because of the geographical scale involved, measures of range size can only be crude, and take no account of areas within the range that lack suitable habitat. They also take no account of the fact that the bulk of the population may occupy only part of the winter range at one time, either shifting south during the course of the northern winter or occurring at any one time only in those parts where rainfall or other factors have created suitable conditions (Chapter 16).

Despite the general correlation between the sizes of breeding and wintering areas, in about 69% of Eurasian-African migrants the breeding range is noticeably

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