Most studies of migratory fuelling in birds have been concerned with fuel deposition before departure, and its replenishment at staging areas. However, some birds appear to depart at normal weight without prior fuel deposition, lose weight during the flight, and make it up after arrival in a staging area - an extreme load-minimising strategy. Without special reserves, small birds could not survive much more than a day without food, but they could travel in this way on short flights lasting a few hours, followed by feeding. However, large birds, such as swans, geese and eagles, can normally survive for many days without food, so in theory they could travel for longer periods without prior fuel deposition. This is especially so for soaring birds, which expend little more energy on migration than on normal daily life. Most soaring species that have been studied accumulate relatively small amounts of fuel for overland flight (for raptors see Chapter 7), and no pre-migratory fattening is apparent in White Storks Ciconia ciconia at either season, despite migrations of up to 10 000 km (Berthold et al. 2001b). These birds feed as they go, mainly in the mornings and evenings, and travel in the middle part of the day, when the thermals that permit soaring-gliding flight are best developed. But they face large stretches of the journey, notably through deserts, when they could not expect to feed for several days. In contrast, cranes also travel partly by soaring-gliding flight but accumulate substantial body reserves (up to one-third of body weight), during the several weeks they spend at favoured stopover sites (for Sandhill Crane Grus canadensis see Krapu et al. 1985).
An absence of pre-migratory fattening might also be expected in those overland species that have to leave their breeding areas as soon as parental commitments permit, in order to get out before environmental conditions deteriorate. It is not hard to imagine, therefore, a continuum of variation in which some birds migrate entirely on prior reserves, others entirely on a continually replenished deficit, and yet others on both. But it is the species that accumulate reserves before migration that have received most attention from researchers.
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