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Journal of Experimental Biology 140,325-344 (1988)
Published by Company of Biologists 1988

Gliding Birds: Descending Flight of the Whitebacked Vulture, Gyps Africanus

VANCE A. TUCKER ¹

¹ Department of Zoology, Duke University, Durham, NC 27706, USA

The air speeds and sinking speeds of birds gliding at equilibrium fall in a performance area when these quantities are plotted against one another. Three curves bound the performance area: (i) a curve for minimum sinking speed at each air speed, (ii) a curve for maximum sinking speed at each air speed, and (iii) a curve dependent on the maximum lift coefficient of the wings. I have discussed curve i in a previous paper. This paper discusses the theory of curves ii and iii, which describe rapid descent in gliding birds.

I used an optical tracking device (an ornithodolite) to measure air speeds and sinking speeds of 16 African white-backed vultures (Gyps africanus Salvadori) descending rapidly from altitudes 200-500 m above the ground. The ornithodolite measured the polar coordinates of a bird's position in space (relative to the ground) and recorded them on magnetic tape.

The vultures had air speeds between 5.4 and 39.lms^-1, and sinking speeds between 0.2 and 8.3ms^-1. Most of the observations fell within the theoretical boundaries of the performance area.

These data are consistent with a maximum lift coefficient of 2.2 for the wings of white-backed vultures.

Key words: gliding performance, gliding theory, maximum lift coefficient, ornithodolite, polar curve, sinking speed, soaring, tracking device

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