Figure 13.10 Relationship between the latitudinal spans of breeding and wintering ranges of west Palaearctic breeding birds. Some species winter entirely in the west Palaearctic and others partly or entirely in Africa. Excludes seabirds. Spots show mean values, and lines show one standard error and one standard deviation on either side of the mean. The lack of relationship in coastal birds can be attributed to the fact that in winter they switch from an areal distribution in inland areas to a linear distribution along coastlines, often with a very wide latitudinal spread. From Newton & Dale (1997).
For the 57 Eurasian-Afrotropical landbird migrants as a whole, however, wintering ranges are, on average, about one-third smaller than breeding ranges, and in some species only parts of the wintering range may be occupied at any one time (Figure 13.9). These findings imply that most species live at greater densities in their African wintering areas than in their Eurasian breeding areas, but whether this reflects differences in available land area, or in the per unit area capacities of the two regions to support the birds at the times they are present remains unknown. It may be that individual birds need more space in Eurasia when they are feeding young than in Africa when they have only themselves to feed. Moreau (1972) estimated that, owing to warmer weather, the individual daily energy needs of passerines in Africa were about 60% of their breeding season needs. Whatever the reason for the differences between sizes of breeding and wintering ranges, a similar phenomenon occurs in the New World, where migrants from large parts of North America concentrate each winter in a relatively small area in the northern Neotropics. On average, then, the geographical ranges of most species are smaller and more overlapping in winter than in summer. The sizes of breeding and wintering ranges of different species are broadly correlated nevertheless.
Among wetland birds (waterfowl and waders) that winter in inland areas, the sizes of breeding and wintering ranges are also correlated, but in contrast to land-birds, wintering ranges are generally larger, and cover a greater latitudinal span (Figure 13.10). This may be because, as wetlands become scarcer southwards (from tundra to savannah), freshwater birds have to spread over greater areas than landbirds in winter in order to find enough suitable habitat. Freshwater birds may also make longer movements within a winter than landbirds, in response to rainfall patterns (Newton & Dale 1997).
It is hard to get an appropriate measure of the availability of shallow (and often temporary) wetlands in Africa, compared to Eurasia. The total annual renewable water resource, calculated as the 'average annual flow of rivers and ground water generated from endogenous precipitation', was given by the World Resources Institute (1994) as 4184 km3 for Africa, compared with 17 219 km3 for Eurasia. This translates to 0.14 km3 per km2 of land area in Africa and 0.32 km3 per km2 of land area in Eurasia, more than a two-fold difference between these land masses. Moreover, greater evaporation in Africa would greatly increase this difference in terms of surface water, and the big variations in rainfall, both from year to year and from place to place, would further contribute to the sporadic nature of much wetland habitat in Africa.
Most shorebird species switch from an areal distribution on the tundra in summer to a linear distribution on coastlines in winter (Newton & Dale 1997). They therefore breed only in a narrow span of latitude, mostly between 70° and 80°N, but in winter extend southwards over 116° of latitude between about 60°N and 56°S, reaching the southern tip of Africa (35°S), Australasia (47°S) or South America (56°S). Some species, such as Ruddy Turnstone Arenaria interpres, can be found in winter on many a rocky coast within this wide latitudinal span, while others, such as Red Knot Calidris canutus, may be restricted to the relatively few sites where suitable conditions occur, but are found there in great numbers. Because of the seasonal switch in habitat, it is difficult to compare the sizes of their breeding and wintering ranges but, as a group, they show no relationship between the latitudinal extents of breeding and wintering ranges (Figure 13.10). Overall, then, the correlation between summer and winter range sizes is most apparent in landbirds.
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