VISUAL PIGMENTS, OIL DROPLETS AND CONE PHOTORECEPTOR DISTRIBUTION IN THE EUROPEAN STARLING (STURNUS-VULGARIS)

Microspectrophotometric measurements of retinal photoreceptors from th e European starling (Sturnus vulgaris) revealed four classes of single cone, containing visual pigments with wavelengths of maximum absorban ce (lambda(max)) at 563, 504, 449 and close to 362 nm, The two longer- wave-sensitive single cones contained brightly coloured oil droplets w hich cut off light below 572 and 514 nm, respectively, The 449 nm lamb da(max) pigment was associated with a 'colourless' oil droplet with pe ak measured absorptance below 400 nm, The ultraviolet-sensitive visual pigment was paired with a transparent oil droplet which showed no sig nificant absorption above 350 nm, A single class of double cone was id entified, both members of which contained the longwave-sensitive (lamb da(max) 563 nm) visual pigment. The principal member of the double con e contained an oil droplet with a topographically variable cut-off wav elength below 471 nm; the oil droplet found in the accessory member wa s only measured in the ventral retina and displayed three distinct pea ks of absorption at approximately 430, 450 and 480 nm, Rod photorecept ors had a lambda(max) at 503 nm, A new polynomial for fitting visual p igment templates to ultraviolet-sensitive visual pigment data is given . Topographic density measurements of the different cone classes were made using Nitroblue-tetrazolium chloride to label selectively bleache d photoreceptors, The two classes of shortwave-sensitive single cone w ere more abundant in the dorsal retina, and longwave-sensitive single cones were notably less abundant in the dorso-temporal region of the r etina, which subserves binocular vision.