Speeds and wingbeat frequencies of migrating birds compared with calculated benchmarks

Sixteen species of birds passing Falsterbo in southwest Sweden during the autumn migration season were observed using short-range optical methods. Air speeds and wingbeat frequencies were measured, reduced to sea level, and compared with benchmark values computed by Flight.bas, a published flight performance program based on flight mechanics. The benchmark for air speed was the calculated sea-level value of the minimum power speed (V(mp)). The mean speeds of three raptor species that flew by flap-gliding were below V(mp), apparently because the flap-glide cycle involved slowing down below V(mp) when gliding and accelerating back up to V(mp) when flapping. The mean speeds of 11 species that flew by continuous flapping were between 0.82V(mp) and 1.27V(mp). Two passerine species that flew by bounding had mean speeds of 1.70V(mp) and 1.96V(mp), but these high mean speeds reflected their ability to fly faster against head winds. These results do not support predictions from optimal migration theory, which suggest that migrating birds 'should' fly faster, relative to V(mp). However, observations were restricted for technical reasons to birds flying below 200 m and may not represent birds that were seriously committed to long-distance migration. The benchmark wingbeat frequency (f(ref)) was derived from dimensional reasoning, not from statistical analysis of observations. Observed wingbeat frequencies ranged from 0.81f(ref) to 1.05f(ref), except in the two bounding species, whose wingbeat frequencies appeared anomalously high. However, the mechanics of bounding with a power fraction q imply that gravity during the flapping phase is increased by a factor 1/q, and when the value of gravity was so adjusted in the expression for f(ref), the wingbeat frequencies of the two bounding species were predicted correctly as a function of the power fraction. In small birds with more muscle power than is required to fly at speeds near V(mp), bounding is an effective method of adjusting the specific work in the muscle fibres, allowing conversion efficiency to be maximised over a wide range of speeds.