Individual birds can vary their flight speed. In species that fly by flapping flight (as opposed to soaring-gliding flight), the energy need is greater at slow and fast speeds than at intermediate speeds. The energy costs of powered (flapping) flight are 8-30 times greater than basal metabolic rate, and of gliding flight about 1.5-2 times greater. While these energy needs depend on speed in flapping flight, they are independent of speed in soaring-gliding flight, in which most energy for flight comes from the ambient atmosphere.
Body size sets constraints on migration in birds. With increasing body mass, flight costs increase, as does flight efficiency (energy cost per unit weight) and flight speed, but the amount of fuel (relative to body weight) that can be carried declines, reducing the maximum possible non-stop flight range. Wing design has additional influence on flight speed and efficiency, but seems influenced as much by the needs of everyday life as by those of migration. With favourable winds, swans (some of the heaviest of flying birds) can make non-stop flights covering up to 1700 km and geese up to 3000 km. Passerines and shorebirds can make non-stop flights of 80-100 hours, enabling passerines to cover distances of 1500-3000 km, and the faster shore-birds distances of 4000-7500 km (with one extreme flight exceeding 10 000 km).
Most bird species migrate by flapping flight, but some large species migrate mainly by soaring and gliding. This method requires much less energy per unit time than flapping flight but, being dependent largely on thermals or other rising air currents, soaring landbird species often have to take roundabout routes avoiding long water crossings. They can also travel only by day, while many other (flapping) birds can travel at night.
While fuel and water could clearly limit the length of non-stop flights by birds, some species may need to rest periodically, well before they reach the limit set by fuel reserves. Species that migrate by walking or swimming travel more slowly and generally shorter distances than species that migrate by flight, but migrations exceeding 1000 km have been recorded from some penguin species.
White Pelicans Pelecanus onocrotalus soaring in an updraft
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