Study of the altitude at which birds migrate is of interest not only to ornithologists, but is also of practical relevance in conservation (because birds can kill themselves against masts and other tall structures) and in aircraft safety (because birds can get caught in jet engines and bring down planes). Over low land, to judge from radar studies, most migration takes place within 1.5 km of the ground, with decreasing numbers of birds at higher altitudes up to 3 km or more (Bruderer 1999). When birds need to cross high mountains or find favourable airstreams, they sometimes fly much higher, occasionally reaching more than 5 km (Figure 4.4). Radar studies
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