Spectral reflectance curves (±1s.e.) of (a) white breast feathers from chickens and (b) yellow breast feathers from American Goldfinches before and after carotenoid extraction with acidified pyridine. The untreated feathers had reflectance curves typical of carotenoid-pigmented barbs whereas pyridine-treated feathers had typical reflectance curves for white feathers (b). n=5 in all cases.

Spectral reflectance curves (±1s.e.) of cresol-treated breast feathers of American Goldfinches (a) against a black background after being chemically de-pigmented (b) against a black background intact, and (c) against a white background intact. Panel A shows that de-pigmented feathers became transparent after cresol treatment, as their reflectance matched that of the black background after being treated with cresol, except for a small increase in overall reflectance. Panel B shows that cresol-treated yellow feathers against a black background are nearly transparent with a faint yellow cast, as indicated by the small increase in brightness from 500 to 700 nm. c shows that cresol-treated yellow feathers against a white background are once again bright yellow, as carotenoids absorb light from the white background. n=5 for all feather reflectance curves. Because all feathers were measured on the same piece of background paper, n=1 for these curves.

(a) TEM micrograph (1900×) of a yellow American goldfinch (Carduelis tristis) feather barb. P, carotenoid pigments; K, keratin substrate; and V, air-filled vacuole. Scale bar, 1 μm. (b) Light micrograph (1000×) of a yellow American goldfinch (C. tristis) feather barb. V, Vacuole. Scale bar, 2 μm.