Photoreceptor spectral sensitivities (relative absorbances) for (a) the colubrid snake, T. sirtalis, and (b) ultraviolet sensitive avian visual system. UVS, ultraviolet sensitive; SWS, short-wavelength sensitive; MWS, medium-wavelength sensitive; LWS, long-wavelength sensitive; D, double cone. Absorbance spectra of the single cones are normalized to equal area under the curve and the bird double cone is normalized to a maximum of 1.