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First published online December 10, 2003
Journal of Experimental Biology 207, 285-293 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00746

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Are melanized feather barbs stronger?

Michael Butler and Amy S. Johnson^*

Biology Department, 6500 College Station, Bowdoin College, Brunswick, ME 04011, USA

^* Author for correspondence (e-mail: ajohnson{at}bowdoin.edu)

Accepted 8 October 2003

Melanin has been associated with increased resistance to abrasion, decreased wear and lowered barb breakage in feathers. But, this association was inferred without considering barb position along the rachis as a potentially confounding variable. We examined the cross-sectional area, breaking force, breaking stress, breaking strain and toughness of melanized and unmelanized barbs along the entire rachis of a primary feather from an osprey (Pandion haliaetus). Although breaking force was higher for melanized barbs, breaking stress (force divided by cross-sectional area) was greater for unmelanized barbs. But when position was considered, all mechanical differences between melanized and unmelanized barbs disappeared. Barb breaking stress, breaking strain and toughness decreased, and breaking stiffness increased, distally along the rachis. These proximal-distal material property changes are small and seem unlikely to affect flight performance of barbs. Our observations of barb bending, breaking and morphology, however, lead us to propose a design principle for barbs. We propose that, by being thicker-walled dorso-ventrally, the barb's flexural stiffness is increased during flight; but, by allowing for twisting when loaded with dangerously high forces, barbs firstly avoid failure by bending and secondly avoid complete failure by buckling rather than rupturing.

Key words: barb, feather, strength, biomechanics, melanin, color, material properties

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