Figure 4.4 Altitudinal distribution of migrants as registered by radar studies in: (a) Lowlands of central Europe, spring (Bruderer 1971); (b) South Sweden, spring, mainly waterfowl and shorebirds (Green 2004); (c) Antigua, Caribbean, autumn (Williams et al. 1977).
in central Europe have also revealed that migration generally occurs at higher elevation in spring than in autumn and by night than by day. However, different patterns of flight altitude occur in other parts of the world, where seasonal wind patterns differ (see Bellrose & Graber 1963 for Illinois, see Klaassen & Biebach 2000 for Sahara Desert).
In some regions, the height of bird migration seems to vary relatively little from night to night, but where wind conditions are highly variable, migration height may change accordingly, with birds flying higher than usual if winds are unfavourable lower down. This was evident, for example, at various sites in the southeastern USA, when migrants were heading towards or from the Gulf of Mexico (Gauthreaux 1991). In this region, migration on many nights was concentrated at less than 500 m above ground, but when winds were unfavourable in this zone, the birds flew higher, giving over a number of nights a clear correlation between the altitude with the greatest density of migrants and the altitude with the most favourable winds (Figure 4.5).
Apart from the needs to find favourable winds or to cross mountains, flights at several kilometres high are likely to occur mainly on long non-stop journeys, such as over oceans. So much energy is consumed in climbing to such high altitudes that this might not be worthwhile on short flights. Migrants from northeastern
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