Gliding flight in a jackdaw: A wind tunnel study

We examined the gliding flight performance of a jackdaw Corvus monedula in a wind tunnel. The jackdaw was able to glide steadily at speeds between 6 a nd 11 ms(-1) The bird changed its wingspan and wing area over this speed ra nge, and me measured the so-called glide super-polar, which is the envelope of fixed-wing glide polars over a range of forward speeds and sinking spee ds. The glide super-polar was an inverted U-shape with a minimum sinking sp eed (V-ms) at 7.4ms(-1) and a speed for best glide (V-bg) at 8.3 ms(-1). At the minimum sinking speed, the associated vertical sinking speed was 0.62 ms(-1). The relationship between the ratio of lift to drag (L:D) and airspe ed showed an inverted U-shape with a maximum of 12.6 at 8.5ms(-1). Wingspan decreased linearly with speed over the whole speed range investigated. The tail was spread extensively at low and moderate speeds; at speeds between 6 and 9 ms(-1), the tail area decreased linearly with speed, and at speeds above 9 ms(-1) the tail was fully furled, Reynolds number calculated with t he mean chord as the reference length ranged from 38 000 to 76 000 over the speed range 6-11 ms(-1). Comparisons of the jackdaw flight performance wer e made with existing theory of gliding flight. We also reanalysed data on s pan ratios with respect to speed in two other bird species previously studi ed in wind tunnels. These data indicate that an equation for calculating th e span ratio, which minimises the sum of induced and profile drag, does not predict the actual span ratios observed in these birds. We derive an alter native equation on the basis of the observed span ratios for calculating,wi ngspan and wing area with respect to forward speed in gliding birds from in formation about body mass, maximum wingspan, maximum wing area and maximum coefficient of lift. These alternative equations can be used in combination with any model of gliding flight where wing area and wingspan are consider ed to calculate sinking rate with respect to forward speed.