FLIGHT STYLE OF THE BLACK-BILLED MAGPIE - VARIATION IN WING KINEMATICS, NEUROMUSCULAR CONTROL, AND MUSCLE COMPOSITION

Black-billed magpies (Pica pica; Corvidae) exhibit an unusual flight s tyle with pronounced, cyclic variation in wingbeat frequency and ampli tude during level, cruising flight. In an effort to better understand the underlying internal mechanisms associated with this flight style, we studied muscle activity patterns, fiber composition of the pectoral is muscle, and wingbeat kinematics using both laboratory and field tec hniques. Over a wide range of speeds in a windtunnel (0-13.4 m s(-1)), wingbeat frequency, wingtip elevation, and relative intensity of elec tromyographic (EMG) signals s(-1) from the flight muscles were least a t intermediate speeds, and increased at both slower and faster speeds, in approximate agreement with theoretical models that predict a U-sha ped curve of power output with flight speed. Considerable variation wa s evident in kinematic and electromyographic variables, but variation was continuous, and, thus, was not adequately described by the simple two-gait system which is currently accepted as describing gait selecti on during vertebrate flight. Indirect evidence suggests that magpies v ary their flight style consistent with reducing average power costs in comparison to costs associated with continuous napping at a fixed lev el of power per wingbeat. The range of variation for the kinematic var iables was similar in the field and lab; however, in the field, propor tionally fewer flights showed significant correlations between wingbea t frequency and the other variables. Average flight speed in the field was 8.0 m s(-1). Average wingbeat frequency was less in the field tha n in the windtunnel, but mean values for wingtip elevation and wingspa n at midupstroke were not significantly different. Histological study revealed that the pectoralis muscle of magpies contained only fast-twi tch (acid-stable) muscle fibers, which were classified as red (R) and intermediate (I) based on oxidative and glycolytic capacities along wi th fiber diameter. This fiber composition may be related to variation in wingbeat kinematics, but such composition is found in the pectorali s of other bird species. This suggests that the muscle fibers commonly found in the pectoralis of small to medium sized birds are capable of a wider range of efficient contractile velocities than predicted by e xisting theory. Future studies should explore alternative explanations for variation in wingbeat kinematics, including the potential role of nonverbal communication among cospecifics. (C) 1997 Wiley-Liss, Inc.