To test for the importance of the structural component of yellow feathers, we first treated yellow American goldfinch feathers and white chicken feathers (which served as controls) with acidified pyridine to remove carotenoids. White chicken feathers increased slightly in achromatic brightness following treatment, whereas the yellow feathers turned white (). This treatment demonstrates that yellow carotenoid feathers have a white background.
Figure 1 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 (more ...)
We next treated feathers with cresol, a liquid with the same RI as keratin that removes the structural component of coloration. Whether de-pigmented or naturally white, white feathers became transparent after cresol treatment (a). Yellow feathers treated with cresol became nearly transparent with a faint yellow cast (b) when placed against a black background. When these cresol-treated yellow feathers were placed against a white background, however, they once again appeared bright yellow (c). On a black background, coloration comes only from the direct reflection of light from the carotenoid pigments and this produces minimal coloration. On a white background, carotenoids absorb reflected light from the white background and again appear bright yellow. We obtained similar results when we applied the same methods to yellow feathers of prothonotary warblers (Prothonotaria citrea), cedar waxwings (Bombycilla cedrorum), and great crested flycatchers (Myiarchus crintus) (M. D. Shawkey, unpublished data). Thus, these results appear to apply to yellow carotenoid-pigmented feathers generally.
Figure 2 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. (more ...)
The yellow feathers of male American Goldfinches have the carotenoid pigments canary xanthophylls a and b (Stradi et al. 1995
; McGraw et al. 2001
), which are purported to produce the yellow feather colour. Here we show, however, that the yellow feathers of goldfinches, and probably of other species, also have white structural coloration that plays a key role in producing their yellow and UV reflectance peaks. Our observations show that carotenoids function primarily to absorb light at wavelengths from approximately 400 nm to 500
nm from the structural white colour of the barbs.
Our microscopic examinations revealed that carotenoid pigments are scattered haphazardly throughout the keratin substrate of feather barbs (a
). This spatial arrangement may allow carotenoids to absorb light from the white keratin substrate while weakly reflecting yellow light, resulting in the pure yellow coloration seen in b
. Thus, carotenoids probably serve as semi-transparent filters in yellow feathers. The mechanisms of white colour production in feather barbs are largely unknown (Prum 1999
), however, and elucidation of these mechanisms by precise physical modelling is needed before we can state definitively how white tissue and carotenoids interact to produce colour. Our results clearly show that both white structural colour and pigments are needed for the production of yellow colour, and these results have several important implications for the study of plumage coloration.
Figure 3 (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, 1μm. (b) Light micrograph (1000×) of a yellow (more ...)
First, while the characterization of colour displays as pigment-based or structural has heuristic value and should continue to be used, researchers should understand that many pigmentary mechanisms have a structural component. Here, we have shown that carotenoid displays rely critically on underlying white structural coloration.
Second, the blurring of the distinction between mechanisms of production may add to the complexity of feather signalling properties. It has been shown that changes in red and yellow coloration, particularly in hue and saturation (i.e. colour purity), are proportional to changes in types and concentrations of carotenoids in the feather (Hill 2002
; Saks et al. 2003
). In turn, birds in poor condition and with greater parasite loads tend to be duller and have lower carotenoid concentration than those in good condition (Hill 2002
). More carotenoids can cover the white structure more completely than fewer carotenoids, resulting in a more saturated and pure colour. Thus, that saturation and purity of carotenoid coloration would dependably reflect pigment concentration, which in turn could signal foraging ability or parasite load (Hill 2002
). Other elements of yellow coloration, however, such as UV chroma may be affected by variation in structural, as well as pigmentary components. Indeed, differences in UV reflectance between male and female yellow-breasted chats (Icteria virens
) cannot be explained by differences in carotenoid concentration alone, and thus are presumably influenced by differences in structural coloration as well (Mays et al. 2004
The brightness, or total reflectance, of carotenoid-containing tissues is probably also strongly influenced by structural components of barbs, as the reflection of light (as opposed to the subtraction of light) is primarily a function of the underlying white structural colour. Recent studies of variation in achromatic brightness of white plumage patches suggest a signalling function for white plumage (Doucet et al. 2005
). The production mechanisms and extent of variation in white coloration, however, remain largely unknown (Mennill et al. 2003
). Clearly, more information on these topics is needed. Perhaps the structural and pigmentary components of feather coloration signal different, or redundant, information that is assessed simultaneously by the receiver. In this way, single feather patches could contain multiple ornaments (Candolin 2003
Finally, these results may partially explain the widespread occurrence of UV reflectance in avian plumage. Ultraviolet reflectance is close to ubiquitous among birds with pigmented feathers (Eaton & Lanyon 2003
). This pattern may partly be caused by the near-ubiquity of structural white plumage colour in carotenoid-pigmented feathers. However, some pigments may reflect in the UV and not all structural white feathers reflect well in the UV, so further testing of this hypothesis is needed. Our results and the white colour of the proximal portions of many pigmented feathers suggest that white structural colour may be found commonly in conjunction with carotenoids and other pigments. Structural colour could be the foundation on which much ‘pigment-based’ plumage colour is based and its presence and variation may have to be taken into account in future studies of avian plumage colour signalling.