Effects of speed and body size on the intermittent flight of birds.

摘要

In contrast to continuous flapping flight, many species of birds exhibit one of two intermittent flight styles known as flap-bounding or flap-gliding. These forms of flight have been considered from a theoretical perspective in the existing literature, but the study described herein represents new empirical observations with the goal of understanding how and why birds use intermittent flight styles. Kinematic and electromyographic data were collected from Budgerigars (Parakeets), Melopsittacus undulatus, representing a typical flap-bounding species and European Starlings (Sturnus vulgaris) representing a typical flap-gliding species. As the birds flew across a range of speeds in a wind tunnel, they exhibited flap-gliding at slow speeds and flap-bounding at high speeds. The percentage of time spent flapping generally decreased at intermediate speeds. These behavior patterns were consistent with minimizing energy expenditure according to aerodynamic theory.; Budgerigars and European Starlings exhibited multiple-spike electromyographic (EMG) bursts during wing flapping that represented motor unit action potential trains or asynchronous twitch contractions from different motor units. The relative intensity of EMG bursts from the primary flight muscles increased with flight speed, probably indicating an increase in average force production. Both species consistently exhibited a periodic rise and fall in EMG bursts during flapping which appeared to be related to changes in flight velocity, acceleration, wing kinematics, and wingbeat frequency. During intermittent glides in both species, the pectoralis exhibited an isometric contraction. The supracoracoideus was inactive during glides in budgerigars but active in starlings. During bounds in both species, both muscles were inactive. Both species used intermittent flight in a manner consistent with reducing energy expenditure relative to continuous flapping rather than as a constraint due to the characteristics of their muscle physiology.; Using laboratory and field techniques, I also studied intermittent flight in six species of woodpeckers (Picidae) ranging in body mass from 27-260 g. During level flight, all of the species exhibited flap-bounding. The number of flaps in a flapping phase, wingbeat frequency, flapping phase duration, and bounding phase duration scaled negatively with body mass. Both percent of the time spent flapping and flight speed showed weak positive scaling relationships with body mass. The Lewis' Woodpecker (Melanerpes lewis) appeared unusual in its intermittent flight behavior by spending more time flapping, using lower wingbeat frequencies, and flying slower than would be expected for its body mass. It was the only species to use highly maneuvered flights, including extended glides to pursue flying insects, although Red-naped Sapsuckers (Sphyrapicus nuchalis) were observed to flycatch without gliding. These two species had slightly more pointed wings than the other species, but contrary to prior reports, external and internal wing dimensions were geometrically similar among species. The pectoralis muscle in woodpeckers less than 100 g in body mass was composed mostly of red (R) fibers with Intermediate (I) fibers either absent or restricted to lateral regions. This scaling of muscle composition may explain why species larger than 100 g flap-bounded when theory predicts they should not have sufficient power. Intermediate fibers in the Lewis' Woodpecker were unique in staining intensely for glycolytic potential--perhaps reflecting their foraging style.; Overall, my results suggest that a given bird species may facultatively change from flap-gliding to flap-bounding and that speed is a dominant factor in determining the selection of either pattern.