Info

Data mainly from Jehl et al. (2003), in which the original references may be found; also Butler & Woakes (1990) for Domestic Pigeon, Baudinette & Schmidt-Nielsen (1974) for Herring Gull. Other estimates of the costs of flapping flight are summarised by Norberg (1996), but not in terms of BMR. a Basal metabolic rate (BMR) is the minimum rate of energy expenditure of an animal at rest, in a post-absorptive state. Note that the flight costs in this table are not directly comparable between species, because BMR values themselves vary with body weight, and in larger birds the chemical power required to fly at Vmr (say) is a larger multiple of BMR than in smaller birds. For their size, the grebes are exceptional in the power required to fly.

b DLW, doubly-labelled water; MC, mass change; PM, Pennycuick's theoretical model; WT, wind tunnel.

Data mainly from Jehl et al. (2003), in which the original references may be found; also Butler & Woakes (1990) for Domestic Pigeon, Baudinette & Schmidt-Nielsen (1974) for Herring Gull. Other estimates of the costs of flapping flight are summarised by Norberg (1996), but not in terms of BMR. a Basal metabolic rate (BMR) is the minimum rate of energy expenditure of an animal at rest, in a post-absorptive state. Note that the flight costs in this table are not directly comparable between species, because BMR values themselves vary with body weight, and in larger birds the chemical power required to fly at Vmr (say) is a larger multiple of BMR than in smaller birds. For their size, the grebes are exceptional in the power required to fly.

b DLW, doubly-labelled water; MC, mass change; PM, Pennycuick's theoretical model; WT, wind tunnel.

it is perhaps surprising that some other large species, such as geese and swans, migrate entirely by flapping (except in unusual circumstances, Klaassen et al. 2004). However, these large waterfowl migrate mainly in regions where thermals are weak at migration times, and their flock formations provide other ways of reducing energy costs. Even small birds, such as warblers, could save energy by soaring if they had the right wing structure, but they would then travel only very slowly (less than half their flapping speed). Moreover, the thermal convection that allows soaring is usually available for only part of each day, and by restricting the daily migration time for soaring flight, assuming time is important, the body size favouring soaring shifts even more towards larger sizes (Hedenstrom 1993).

Any columns of rising air must be embedded in wider areas of more slowly sinking air, which replace the rising air. This means that, regardless of body size and flight mode, any migrating bird could gain energy (in the form of net lift) if it flew more slowly through updrafts than between them, the resulting energy gain being subsequently translated into increased cross-country speed or flight range (Nisbet 1962). On this basis, rising air currents could benefit all birds, not just soaring species, and may be of special value to species that travel long distances over hot deserts. It is not, of course, necessary for an air current to be strictly vertical to permit soaring, so long as it has a vertical component that exceeds the bird's own rate of sink.

0 0

Post a comment