Flying Along a Logarithmic Spiral

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Science  08 Dec 2000:
Vol. 290, Issue 5498, pp. 1857
DOI: 10.1126/science.290.5498.1857c

Peregrine falcons use two high performance skills—flight speed and visual acuity—to attack their prey. In a trio of papers, Tucker and his colleagues model the quantitative aspects of aerodynamic drag, assess the optimal integration of speed and sight, and present field observations of falcon flight. Drag, particularly at flight speeds in excess of 50 meters per second, is minimized by keeping the head aligned with the body, but this presents a problem: prey of the size of a robin can be seen from a distance of 1 km only by using the deep fovea, which is oriented at an angle of about 40 degrees from the head axis. Thus, it would be impossible for a falcon to see its prey while flying straight at it, and keeping the head angled while flying would increase drag significantly and reduce speed. The solution is for the falcon to fly along a logarithmic spiral, with head and body aligned along the line-of-flight at a 40 degree angle from line-of-sight to the prey, until almost 90 % of the distance has been covered, at which point binocular acuity is sufficient to guide straight-line attack. Several summers of observation with computerized tracking equipment at Deer Valley Ranch, Colorado confirm that peregrine falcons do indeed fly along spiral paths. — GJC

J. Exp. Biol.203, 3733; 3745; 3755 (2000).

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