WINGBEAT FREQUENCY AND THE BODY DRAG ANOMALY - WIND-TUNNEL OBSERVATIONS ON A THRUSH NIGHTINGALE (LUSCINIA-LUSCINIA) AND A TEAL (ANAS-CRECCA)

A teal (Anas crecca) and a thrush nightingale (Luscinia luscinia) were trained to fly in the Lund wind tunnel for periods of up to 3 and 16 h respectively. Both birds flew in steady flapping flight, with such r egularity that their wingbeat frequencies could be determined by viewi ng them through a shutter stroboscope. When flying at a constant air s peed, the teal's wingbeat frequency varied with the 0.364 power of the body mass and the thrush nightingale's varied with the 0.430 power. B oth exponents differed from zero, but neither differed from the predic ted value (0.5) at the 1 % level of significance. The teal continued t o flap steadily as the tunnel tilt angle was varied from -1 degrees (c limb) to +6 degrees (descent), while the wingbeat frequency declined p rogressively by about 11 %. In both birds, the plot of wingbeat freque ncy against air speed in level flight was U-shaped, with small but sta tistically significant curvature. We identified the minima of these cu rves with the minimum power speed (V-mp) and found that the values pre dicted for V-mp, using previously published default values for the req uired variables, were only about two-thirds of the observed minimum-fr equency speeds. The discrepancy could be resolved if the body drag coe fficients (C-Db) of both birds were near 0.08, rather than near 0.40 a s previously assumed. The previously published high values for body dr ag coefficients were derived from wind-tunnel measurements on frozen b ird bodies, from which the wings had been removed, and had long been r egarded as anomalous, as values below 0.01 are given in the engineerin g literature for streamlined bodies. We suggest that birds of any size that have web-streamlined bodies can achieve minimum body drag coeffi cients of around 0.05 if the feet can be fully retracted under the fla nk feathers. In such birds, field observations of flight speeds may ne ed to be reinterpreted in the light of higher estimates of V-mp. Estim ates of the effective lift:drag ratio and range can also be revised up wards. Birds that have large feet or trailing legs may have higher bod y drag coefficients. The original estimates of around C-Db=0.4 could b e correct for species, such as pelicans and large herons, that also ha ve prominent heads. We see no evidence for any progressive reduction o f body drag coefficient in the Reynolds number range covered by our ex periments, that is 21 600-215 000 on the basis of body cross-sectional diameter.