The various migrations described in this chapter indicate what some birds are capable of achieving on their annual travels. The journeys involved may be assumed to push birds to the limits of their endurance: long non-stop flights without sleep, rest, food or water. Most involve high fuel deposition beforehand, and special adaptations for high-altitude flight, extreme heat or cold, dehydration, low air densities and oxygen levels - conditions in which few animals could survive. The fact that some birds can achieve these high endurance feats without prior training or acclimatisation is all the more remarkable.
Many of the findings reported in this chapter have come to light in the last 25 years, but more research is needed to better understand the physiological mechanisms involved. For example, humidity has so far been considered only in terms of dry conditions which lead to excessive water loss, but the humid conditions over tropical rainforests could bring the opposite problem, and reduce the effectiveness of evaporative cooling. This could greatly raise the optimal altitude for flight.
It is uncertain how often birds need to rest while on migration, or how far they could fly without resting (Chapter 3). Resting is of course not normally possible for landbirds migrating over water, and continuous wing-beating for periods up to 70 hours or more seems incredible for small birds. Many birds seem 'tired' after overwater flights, in that they flop down as soon as they reach a place to stop, fly only with reluctance if disturbed, and can be seen to sleep in the daytime. Typically, on arrival, they drink and then sleep (Schwilch et al. 2002b). Many other birds stop for periods of a few hours, apparently without feeding, even where food is available (for further discussion, see Chapter 5). Large waterfowl are known from radio-tracking to sit on the sea, hundreds of kilometres from land. Such birds may be resting or simply sitting out periods of unfavourable weather. The same is true for seabirds which migrate by dynamic soaring, but which in calm conditions sit on the sea and wait for the wind to get up.
Depending on their journeys, birds need fuel and water in different proportions, and on non-stop flights when they can neither feed nor drink, birds can address these different needs physiologically or behaviourally. They may adjust the ratios of stored and metabolised fat, protein and carbohydrate, so that body reserves yield different ratios of energy and water. Alternatively, birds might adjust the times of day (day or night), or the altitudes at which they fly, both of which influence rates of heat and water loss. Birds can also reduce their energy needs by selecting times or altitude zones with favourable winds (Chapter 4). One of the most striking findings from studies of difficult journeys is how much birds use behavioural means to overcome difficulties, involving, for example, choice of wind conditions, flight altitudes and flight times, as well as stopover frequency and duration, and whether to feed or rest during stopovers. Some types of behaviour are appropriate in some conditions but not in others. For example, the burning of body fuel for flight releases heat, which helps to maintain body temperature in cold conditions (high latitudes and altitudes), but could lead to overheating (and hence water loss) in warm conditions (low latitudes and altitudes). Hence, in order to maintain a favourable energy and water balance, a bird must continually adjust its flight times, flight altitudes and other behaviour according to prevailing conditions. In these various ways, a bird can not merely survive, but can maximise the time and distance it can fly on a given fuel load.
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