Colour Discrimination of Two Passerine Bird Species in the Munsell System

Abstract

Two male Yellow Wagtails Motacilla f. flava and one male Bluethroat Luscinia s. svecica were successively trained to ten colour papers, each representing one of the ten colour families in the Munsell 40 hue system. Discrimination was tested between the training hue and four neighbouring hues under standardized illumination. Yellow Wagtail hue discrimination was best in the red, orange and yellow families and less good towards shorter wavelengths and for red-purple hues. The Bluethroat discriminated best in the orange, purple-blue and purple families. Both species could memorize and make statistically significant discriminations between c. 20 hues. Their best discrimination occurred in spectral regions corresponding to their own sexually dimorphic plumage. Brightness (value) and saturation (chroma) discrimination was studied by training the same birds to the middle value/chroma of seven Munsell hues. The birds' value discrimination was poor: they learned to reject the darkest value(s) but not the brighter ones. Both species could discriminate between the less saturated chromas but they chose more saturated colours almost as frequently as the correct ones. Between the species there were differences particularly in chroma discrimination similar to those in hue discrimination. The Bluethroat's better hue discrimination in the short wavelength end of the spectrum (violet and blue) may tentatively be connected with the fact that this species has a larger proportion of cones sensitive to shorter wavelengths (about 75% of cones with colourless or pale greenish droplets), while in the Yellow Wagtail about 50% of the cones have red or yellow-orange oil droplets transmitting light efficiently only in the long wavelength part of the spectrum.