Visual pigments and oil droplets in the retina of a passerine bird, the canary Serinus canaria: microspectrophotometry and opsin sequences.

The visual receptors of the passeriform bird Serinus canaria, the canary, have been examined microspectrophotometrically and the sequences of the opsins determined. Rods have a maximum absorbance (lambda max) at 506 nm. Four spectral classes of single cone are present: long-wave-sensitive (LWS) containing a photopigment with lambda max at 569 nm, middle-wave-sensitive (MWS) with lambda max at 505 nm, short-wave-sensitive (SWS) with lambda max at 442 nm, and ultraviolet-sensitive (UVS) with lambda max at about 366 nm. Double cones possess the 569-nm pigment in both members. Typical combinations of photopigment and oil droplet occur in most cone classes. An ambiguity exists in the oil droplet of the single LWS cones. In some birds, LWS cones are paired with an R-type droplet, whereas in the majority of canaries the LWS pigment is paired with a droplet similar to the P-type of double cones. Mechanisms of spectral tuning within each opsin class are discussed.