Morphology of UV reflectance in a whistling-thrush: implications for the study of structural colour signalling in birds

Many birds perceive, and some seem to communicate, in the W-A waveband (320-400 nm) invisible to humans, but the morphological mechanism and thereby scope for ultraviolet plumage signalling remain largely unstudied. No avian pigments or iridescence that reduce or enhance primarily UV reflectance are known, but a UV version of the common Tyndall "spongy structure" has been found in a parrot. A similar UV structure is here described for the first time in a passerine, the Blue Whistling-thrush Myiophonus caeruleus. Among patches of its plumage, and across other investigated species, a correlation between spongy structure dimensions and "hue" (spectral location of the reflectance peak) is consistent with current models of the optical mechanism, in particular the "hollow cylinder model" by Dyck (1971). Variations in "brightness" (intensity) and "chroma" (spectral purity) seem due mainly to modifications of ramus shape (flattening and a unique type of lateral flanges) and spongy layer thickness, here quantified by light paths laid out on the cross sections. The results suggest a wide potential for UV signals in passerines, but pure ("hidden") UV ornaments might be rare for other reasons. Keratin absorbance and declining ambient light in this spectral range: and perhaps also the 370 nm sensitivity maximum of the avian UV receptor, might constrain UV signals to the 400 nm region where they will be perceived as conspicuously violet also by humans. Finally, some possible "honesty-maintaining" ontogenetic costs of UV/violet plumage colours are discussed.