Vorobyev, M., Osorio, D., Bennett, A., Marshall, N., & Cuthill, I. (1998).
 Tetrachromacy, oil droplets and bird plumage colours. Journal Of Comparative
 Physiology A-Neuroethology Sensory Neural And Behavioral Physiology, 183(5), 621-633.
Hart, N. S. (2001). The visual ecology of avian photoreceptors.
 Progress In Retinal And Eye Research, 20(5), 675-703.
Stoddard, M. C., & Prum, R. O. (2008). Evolution of avian plumage
 color in a tetrahedral color space: A phylogenetic analysis of new world buntings.
 The American Naturalist, 171(6), 755-776.
Endler, J. A., & Mielke, P. (2005). Comparing entire colour patterns
 as birds see them. Biological Journal Of The Linnean Society, 86(4), 405-431.