... induced drag non-planar wing primary feathers soaring span efficiency factor tip feathers vortex diffusion vortex spreading winglets Harris’ hawk Parabuteo unicinctus Most birds that soar over land have prominent, separated primary feathers at the tips of their wings. These feathers...

... and different wing tips. The feathered wing tip was slotted and was made of four primary feathers from a Harris’ hawk ( Parabuteo unicinctus ). The Clark Y tip was unslotted and was made of balsa wood shaped to a Clark Y aerofoil. The balsa feather tip was slotted and was made of three balsa wood wings shaped...

... of Bristol, Woodland Road, Bristol BS8 1UG, England. † To whom reprint requests should be addressed. 20 12 1991 © 1992 by Company of Biologists 1992 bird wing drag wind tunnel Parabuteo unicinctus The profile drag of the wings is one of three major components...

... Parabuteo unicinctus Gliding birds typically change their wing spans while flying, both in nature and in wind tunnels ( Pennycuick, 1968 ; Tucker and Parrott, 1970 ; Parrott, 1970 ; Tucker and Heine, 1990 ). This behaviour theoretically minimizes drag ( Tucker, 1987 ), but it may also change...