Landbirds migrating over the sea must often make long non-stop flights, without rest, food or water. Many birds fly non-stop for periods exceeding 60 hours. Such flights take passerines over distances up to 3000 km, and shorebirds over 40006000 km. The longest non-stop flight known from any landbird species involves one population of Bar-tailed Godwits Limosa lapponica baueri, which travels for more than 175 hours non-stop in autumn, covering more than 10 400 km over the Pacific Ocean from Alaska or Siberia to New Zealand. Many other shore-birds make journeys of up to 6000 km over water, and hundreds of species cross shorter stretches of up to 1500 km, including those that regularly migrate over the Mediterranean Sea or the Gulf of Mexico.
The most arduous desert journeys are undertaken by the many species that cross the most barren parts of the Sahara Desert each spring and autumn, where no food or water are taken en route. Some species are thought to cross the Mediterranean Sea and Sahara Desert together in a single non-stop flight of 15002500 km; others break their journey to refuel in North Africa; while yet others migrate only at night, and stop in the desert during the day, remaining motionless in shady places, again without feeding or drinking. Individuals of the same species may show more than one of these different strategies. Only small proportions of birds stop at oases, but more migrate down the west and east sides of the Sahara, where conditions are better than across the central sector. Analyses of the body composition of migrants caught or found dead on the ground suggest that some may be limited on their desert crossing by energy needs, and others by water needs.
While some birds migrate around mountain ranges, others cross even the highest ranges, including the Himalayas. The main problems are the climb to high altitudes (especially for large birds), and the extreme cold, low air densities and oxygen levels found there. In spring, mountains are also often snow-covered, affording no opportunity to feed. Extreme high-altitude flights (more than 7 km above sea level) have been recorded from Bar-headed Geese Anser indicus that cross the Himalayas. These birds have special adaptations enabling them to extract and utilise efficiently the reduced oxygen levels at high altitudes.
Extreme migrations often involve extreme adaptations of high fuel deposition, water conservation and respiratory physiology, as well as behavioural adjustments in flight times, flight altitudes, stopover frequency and duration, and resting or feeding behaviour.
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