Speeds and wingbeat frequencies of migrating birds

Hedenström and Alerstam (Hedenström and Alerstam, 1995) argued that different optimal air speeds can be identified for a migrating bird depending on whether the ‘currency’ that the bird is trying to maximise is distance per unit fuel energy used, average speed over one or several flight stages and stopovers, or various other possibilities. On the other hand, Alerstam and Hedenström (Alerstam and Hedenström, 1998) conceded that it is difficult in practice to determine whether a migrating bird is attempting to maximise one currency rather than another, by comparing the observed speed with alternative theoretical optimal speeds. Not the least of the difficulties is that, to estimate the theoretical speeds, a curve of power versus speed must first be calculated for the bird. This requires some morphological data, which are never available for each individual bird observed in the field. Species means can be used but, in the case of radar observations, even the species is often doubtful. Even when accurate data are available for body mass and wing measurements, as in the case of a bird flying in a wind tunnel, the exact shape of the power curve at high speeds is poorly known. Recognising that this is critical for estimating optimum speeds, Alerstam (Alerstam, 2000) suggested a way in which the power curve might be modified to cover higher speeds, but more experimental evidence is needed before a revised model could be used with confidence.