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First published online May 8, 2007
Journal of Experimental Biology 210, 1742-1751 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.001701

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Aerodynamics of wing-assisted incline running in birds

Bret W. Tobalske^1,* and Kenneth P. Dial²

¹ Department of Biology, University of Portland, 5000 North Willamette Boulevard, Portland, OR 97203, USA
² Flight Laboratory, Division of Biological Sciences, University of Montana, 32 Campus Drive, Missoula, MT 59812, USA

View larger version (43K): [in this window] [in a new window]   Fig. 1. Wing morphology in three age classes of chukar Alectoris chukar: babies (6–8 d.p.h.), juveniles (25–28 days), and adults (45+ days), which were used to study WAIR and flight. The wings are scaled so that the musculoskeletal portions are the same length. Adapted from Dial et al. (Dial et al., 2006).

View larger version (8K): [in this window] [in a new window]   Fig. 2. Representation of the cross-section of a vortex in the wake of a bird engaged in WAIR including some of the variables measured in the wake vortices. Uind, induced velocity in the momentum jet; h, impulse angle of wake vortex relative to horizontal; s, impulse angle of wake vortex relative to substrate; ß, inclination angle of vortex; S, width of vortex.

View larger version (75K): [in this window] [in a new window]   Fig. 3. Method used for identifying vortex cores and measuring circulation () in the wake of chukar Alectoris chukar as revealed using DPIV in a parasagittal plane at the mid-wing. In this instance, a juvenile chukar, no longer visible in the image, used WAIR to move upwards and toward the right through the illumination plane at an angle of 80°. Movements of its wings left vorticity () in the wake that was concentrated into two presumptive vortex loops, one per downstroke (D1 and D2), which were transected by the illumination plane. (A) Velocity vectors, expressed relative to average velocity, and with backround . (B) Streamlines associated with the vector field. Broken lines indicate regions sampled for (blue=starting vortex, red=ending vortex). (C) Above-threshold – (blue) and (D) above threshold + (red) was integrated with respect to area to measure contiguous circulation ( and ) or all circulation presumed to be associated with the core ( and ).

View larger version (122K): [in this window] [in a new window]   Fig. 4. Velocity fields in the wake of chukar Alectoris chukar during WAIR as revealed using DPIV. Backgrounds in (A,C,E) illustrate the bird and incline and in (B,D,F) represent vorticity (). Transparent red loops represent an assumed 3D shape of vortex rings as inferred from concentrations of about and in the DPIV images. (A,B) Baby, ascending at 65°. (C,D) Juvenile ascending at 90°. (E,F) Adult, ascending at 80°. Both starting and ending vortex cores, evident as concentrations of – and +, respectively, are visible for the wake of the juvenile, whereas the wings and body mask ending vortices in baby and adult. Erroneous vectors due to the DPIV algorithm are apparent above the back of the adult in E.

View larger version (56K): [in this window] [in a new window]   Fig. 5. Velocity fields in the wake of an adult chukar Alectoris chukar during ascending flight at 80°, revealed using DPIV. Backgrounds illustrate the bird (A) and vorticity, (B). Transparent red loops in A represent an assumed 3D shape of vortex ring as inferred from concentrations of about and . Effects of two downstrokes are shown, with starting and ending vortex cores evident as concentrations of – and +, respectively, in B.

View larger version (41K): [in this window] [in a new window]   Fig. 6. Velocity fields, revealed using DPIV, of the wake of a baby chukar Alectoris chukar, 6 d.p.h., which was obviously struggling during WAIR at 65°. Backgrounds illustrate the bird (A) and vorticity, (B). Transparent red loop in A is an assumed 3D shape of vortex ring as inferred from concentrations of about and . Only a starting vortex () is visible. Compared with the wake of the baby in Fig. 3A,B (day 7, post-hatching), the wake is less organized and negative circulation () is less (40.5%).

View larger version (15K): [in this window] [in a new window]   Fig. 7. Normalized circulation (A) and an estimate of average lift relative to weight (B) in different age classes of chukar Alectoris chukar engaged in WAIR and in adults during ascending flight. Values are means ± s.d. (N=2 babies, 5 juveniles, 2 adults).