Visual pigments and colour vision in a nocturnal bird, Strix aluco (tawny owl)

Microspectrophotometer measurements of the oil droplets and visual pigments in the receptors of the tawny owl (Strix aluco) have demonstrated the presence of at least four types of oil droplets and three visual pigments. The majority of cones contain a pale yellow droplet and a yellow-absorbing visual pigment withλ_max 555 nm. Two further types of cone can be recognised containing either a green-absorbing pigment,λ_max 503 nm, or a blue-absorbing pigment,λ_max about 463 nm. A fourth type of cone containing a red oil droplet was also identified. The rods contain a P503, similar in absorbance to the green-absorbing cone visual pigment. From the transmission characteristics of the oil droplets and the absorbance spectra of the cone visual pigments, the effective spectral sensitivities of each cone type have been derived. These photopic mechanisms are discussed in relation to psychophysically-determined photopic sensitivities of the tawny owl and to the photopic systems of two diurnal birds.