Mechanical power curve measured in the wake of pied flycatchers indicates modulation of parasite power across flight speeds

Abstract

<jats:p> How aerodynamic power required for animal flight varies with flight speed determines optimal speeds during foraging and migratory flight. Despite its relevance, aerodynamic power provides an elusive quantity to measure directly in animal flight. Here, we determine the aerodynamic power from wake velocity fields, measured using tomographical particle image velocimetry, of pied flycatchers flying freely in a wind tunnel. We find a shallow U-shaped power curve, which is flatter than expected by theory. Based on how the birds vary body angle with speed, we speculate that the shallow curve results from increased body drag coefficient and body frontal area at lower flight speeds. Including modulation of body drag in the model results in a more reasonable fit with data than the traditional model. From the wake structure, we also find a single starting vortex generated from the two wings during the downstroke across flight speeds (1–9 m s <jats:sup>−1</jats:sup> ). This is accomplished by the arm wings interacting at the beginning of the downstroke, generating a unified starting vortex above the body of the bird. We interpret this as a mechanism resulting in a rather uniform downwash and low induced power, which can help explain the higher aerodynamic performance in birds compared with bats. </jats:p>

Speed consensus and the ‘Goldilocks principle’ in flocking birds (Columba livia)

“…Johansson et al, 2018;Torre-Bueno & Larochelle, 1978). These latter studies suggest, at least in some species, that for intermediate flight speeds, flight costs remain relatively stable (Johansson et al, 2018;Torre-Bueno & Larochelle, 1978). Nevertheless, most studies demonstrating flat power output only measure a speed range of ca.…”

“…The models that predict U-shaped power curves, while the best supported Pennycuick, 1968;Tobalske et al, 2003), are contested by some findings (e.g. Johansson et al, 2018;Torre-Bueno & Larochelle, 1978). These latter studies suggest, at least in some species, that for intermediate flight speeds, flight costs remain relatively stable (Johansson et al, 2018;Torre-Bueno & Larochelle, 1978).…”

“…Flying faster in active flight necessarily costs more (Hedenström, 2009;Tobalske et al, 2003); an increased work rate of muscles is required to increase flap frequency or wingbeat amplitude to achieve faster speeds (Butler, 2016;Hedenström, 2009). Similarly, flying slower than an individual's optimum speed has also been shown to increase work rate, as the momentum of flight provides lift (Heerenbrink et al, 2015;Johansson et al, 2018).…”

Bird wings act as a suspension system that rejects gusts

“…Birds may cope with gusts in a similar way, but the response times and dynamics by which they deal with aerodynamic perturbations are not known, as simultaneous measurements of both the bird's kinematics and the properties of the unsteady airflows they encounter are required. Most previous studies of bird flight mechanics (but see [8]) have taken place in steady laboratory conditions, such as corridors [9][10][11], or wind tunnels [12][13][14][15][16][17], or outdoors [18][19][20][21], where the moment-to-moment variation in the wind that the birds encountered could not be quantified.…”

High aerodynamic lift from the tail reduces drag in gliding raptors

“…1; Movie 1). Application of automated Lagrangian particle tracking velocimetry (see Movie 2) to the study of bird flight is novel, though seeding the air with helium bubbles builds upon the early studies of animal flight (Spedding et al, 1984;Spedding, 1987); and wakes have been measured using smoke and particle image velocimetry for a range of considerably smaller flapping (Spedding et al, 2003;Warrick et al, 2005;Van Griethuijsen et al, 2006;Tobalske et al, 2009;Altshuler et al, 2009;Johansson et al, 2018) and gliding (Henningsson and Hedenström, 2011;Henningsson et al, 2014;KleinHeerenbrink et al, 2016) birds.…”