Analysing
the variation in flight speed among bird species is important in
understanding flight. We tested if the cruising speed of different
migrating bird species in flapping flight scales with body mass and
wing loading according to predictions from aerodynamic theory and to
what extent phylogeny provides an additional explanation for variation
in speed. Flight speeds were measured by tracking radar for bird
species ranging in size from 0.01 kg (small passerines) to 10 kg
(swans). Equivalent airspeeds of 138 species ranged between 8 and 23
m/s and did not scale as steeply in relation to mass and wing loading
as predicted. This suggests that there are evolutionary restrictions to
the range of flight speeds that birds obtain, which counteract too slow
and too fast speeds among bird species with low and high wing loading,
respectively. In addition to the effects of body size and wing
morphology on flight speed, we also show that phylogeny accounted for
an important part of the remaining speed variation between species.
Differences in flight apparatus and behaviour among species of
different evolutionary origin, and with different ecology and flight
styles, are likely to influence cruising flight performance in
important ways.
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