Spectral filter of avian retina cone cells (SWS2) together with dihydrogalloxanthin (Ref.676).
Ref.676 : Matthew B Toomey et al., eLif, v.5, 2016, PMC4947394 "Complementary shifts in photoreceptor spectral tuning unlock the full adaptive potential of ultraviolet vision in birds", DOI: 10.7554/eLife.15675.001.