355 The aerodynamic pressures, forces and power requirements of bird flight are of interest both to the engineer aiming to make flying machines, and to biologists aiming to understand either the physiology of muscle, or bird behaviour and ecology. While soaring and gliding flight can be understood within the context of conventional fixed-wing aerodynamics, appropriate analysis of flapping flight is more uncertain. Pennycuick (1975, 1989) describes adaptations of conventional fixed-wing theory for power calculations of relatively fast flapping flight, and identifies that, at lower speeds, such models are certain to run into difficulties. Pennycuick’s extension of fixed-wing theory for fast flapping flight has the great merit of being somewhat predictive in nature: the power requirements for a bird flying under imagined conditions can be calculated, and so aerodynamic powers as functions of speed or loading – key factors when considering the ecology and behaviour of birds – can be predicted. Power models considering vortex structures have been developed for animal flight (Rayner, 1979a, 1993; Ellington, 1984). Rayner develops power models based on a view of vortex structures derived from wake visualisations (e.g. Spedding et al., 1984; Spedding, 1986, 1987). However, more recent wake visualisation experiments using Digital Particle Image Velocimetry (DPIV) for bird flight under highly controlled wind-tunnel conditions (Spedding et al., 2003a,b) suggest that quite complex wake vortex structures must be considered before appropriate force balances – including the support of body weight – are achieved. Thus, while methods based on assumed vortex structures have provided one route for extending power calculations for flapping flight to slower speeds, direct methods for calculating aerodynamic powers are most appealing. Blade-element techniques, found to be effective for propellers and helicopters, have been extended to flapping flight for hovering (Osborne, 1951; Ellington, 1984; Usherwood and Ellington, 2002a,b) and ascending (Wakeling and Ellington, 1997; Askew et al., 2001) bird and insect flight. However, these techniques are reliant on the knowledge of appropriate values of lift and drag coefficients for each, or some form of average, spanwise wing section or ‘element’. These coefficients can be determined, even for revolving wings (in which case they can be quite different from steadily translating wings; Usherwood and Ellington, 2002a,b), given accurate information on wing shape and, critically, the speed and angle of incidence of the local air. While these details may be found for birds during fast flight within the confines of a wind tunnel (Hedrick et al., 2002), during which locally The Journal of Experimental Biology 208, 355-369 Published by The Company of Biologists 2005 doi:10.1242/jeb.01359 Differential pressure measurements offer a new approach for studying the aerodynamics of bird flight. Measurements from differential pressure sensors are combined to form a dynamic pressure map for eight sites along and across the wings, and for two sites across the tail, of pigeons flying between two perches. The confounding influence of acceleration on the pressure signals is shown to be small for both wings and tail. The mean differential pressure for the tail during steady, level flight was 25.6·Pa, which, given an angle of attack for the tail of 47.6°, suggests the tail contributes 7.91% of the force required for weight support, and requires a muscle- mass specific power of 19.3·W·kg –1 for flight to overcome its drag at 4.46·m·s –1 . Differential pressures during downstroke increase along the wing length, to 300–400·Pa during take-off and landing for distal sites. Taking the signals obtained from five sensors sited along the wing at feather bases as representative of the mean pressure for five spanwise elements at each point in time, and assuming aerodynamic forces act within the x–z plane (i.e. no forces in the direction of travel) and perpendicular to the wing during downstroke, we calculate that 74.5% of the force required to support weight was provided by the wings, and that the aerodynamic muscle-mass specific power required to flap the wings was 272.7·W·kg –1 . Key words: aerodynamics, bird, pigeon, Columba livia, flight, power, lift, pressure, flapping. Summary Introduction Dynamic pressure maps for wings and tails of pigeons in slow, flapping flight, and their energetic implications James R. Usherwood*, Tyson L. Hedrick, Craig P. McGowan and Andrew A. Biewener Concord Field Station, Harvard University, 100 Old Causeway Road, Bedford, MA 01730, USA *Author for correspondence at present address: Structure and Motion Laboratory, The Royal Veterinary College, Hawkshead Lane, North Mymms, Hatfield AL9 7TA, UK (e-mail: jusherwood@rvc.ac.uk) Accepted 26 October 2004 THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY