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Rapid, reversible response of iridescent feather color to ambient humidity

Chad M. Eliason and Matthew D. Shawkey

Open Access Open Access

Abstract

Colorful traits can vary in response to neural control, hormone levels, reproductive state, or abiotic factors. In birds, colorful plumage traits are generally considered static ornaments that only vary irreversibly due to abrasion, bacterial degradation, or wear. However, in this work it is shown that iridescent feather color varies rapidly and reversibly in response to changes in ambient humidity. Based on optical models and sorption experiments, these changes appear to be caused by a swelling of the outer keratin cortex following water absorption. To our knowledge, this is the first study describing dynamic color changes in any keratinous biophotonic nanostructure.

©2010 Optical Society of America

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Figures (4)
Fig. 1
Fig. 1 TEM cross-section of an iridescent T. bicolor feather barbule used to predict reflectance (b). Iridescent tree swallow feather (a) and magnified barbules (inset) showing plane of cross-section (white line perpendicular to barbules). Representative barbule cross-section (b) shows melanosomes (black) and keratin cortex (grey region surrounding melanosomes). Scale bar = 500 nm. Letters correspond to dorsal (d) and ventral (v) surfaces. Measured (solid line) and predicted (dashed line) reflectance based on thin-film optical model (c).

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Fig. 2
Fig. 2 Reversible color change in tree swallow feathers. Reflectance of feathers at 0% RH (solid line) and after an abrupt increase to 80% RH (dashed line) [a]. Representative response of λmax to abrupt increases (vertical dashed line, t = 2 sec) and decreases (vertical dashed line, t = 304 sec) in humidity (b). Points connected with grey line to aid in trend visualization. Inset shows plot of hue vs. time1/2 and linear regression (solid line) for times under 70 sec.

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Fig. 3
Fig. 3 Potential mechanisms for the observed color change. Observed mean reflectance change of tree swallow feathers (n = 9) before (black line) and after (red line) humidity change from 20 to 80% RH (insets are images of a single feather taken at 0% and 100% RH). Model predictions before (black line) and after humidity increases (red line) for keratin swelling (b), surface adsorption (c), and increases (solid line) or decreases (dashed line) in the refractive index of keratin (d). Insets are schematic drawings of barbule cross-sections showing melanosomes (black), keratin cortex (grey), and water molecules (blue circles). Drawings not to scale, allowing for easier visualization of mechanisms.

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Fig. 4
Fig. 4 Variation in color response of different color-producing mechanisms. Filled circles show mean hue change before and after humidity changes and error bars show ± 2 SE. Hue was calculated as λmax in bluebirds and swallows and as wavelength of maximum positive slope in house finches (see Materials & methods for details). Overall effect of species on hue change was significant (ANOVA, F2,24 = 49.21, P<0.0001), and all pair-wise comparisons were significant (Tukey tests, P<0.05).

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