Lofty mountain ranges and wide belts of desert are traversed, and lesser or vaster expanses of sea are crossed. (William Eagle Clark 1912.)
Some birds make what seem to us astounding journeys over seas, deserts, high mountains, or other hostile terrain. This chapter focuses on some of these difficult journeys, and the various adaptations that enable birds to complete them successfully. The fact that some birds can make such journeys does not imply that all could do so. Much depends on features of the bird itself, such as its size and physiology, and also on the habitat to which it is adapted. Oceans are inhospitable for land-birds, continents for pelagic seabirds, open country for forest birds, forests for open country birds and barren deserts for almost all birds. And on some overland routes, waterfowl and waders may encounter few sites where they can rest and feed.
Most flying birds can migrate over small areas of hostile habitat that they can cross in a few hours. The difficulties come mainly on longer journeys which require more than 24 hours of non-stop flight, or entail physiologically harsh conditions, such as temperature and humidity extremes, or greatly reduced oxygen levels.
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