Visual pigments, oil droplets, ocular media and cone photoreceptor distribution in two species of passerine bird: the blue tit (Parus caeruleus L.) and the blackbird (Turdus merula L.)

The spectral absorption characteristics of the retinal photoreceptors of th e blue tit (Pal trs caeruleus) and blackbird (Turdus merula) were investiga ted using microspectrophotometry. The retinae of both species contained rod s, double cones and four spectrally distinct types of single cone. Whilst t he visual pigments and cone oil droplets in the other receptor types are ve ry similar in both species, the wavelength of maximum sensitivity (lambda(m ax)) of long-wavelength-sensitive single and double cone visual pigment occ urs at a shorter wavelength (557 nm) in the blackbird than in the blue tit (563 nm). Oil droplets located in the long-wavelength-sensitive-single cone s of both species cut off wavelengths below 570-573 nm, theoretically shift ing cone peak spectral sensitivity some 40 nm towards the long-wavelength e nd of the spectrum. This raises the possibility that the precise lambda(max ) of the long-wavelength-sensitive visual pigment is optimised for the visu al function of the double cones. The distribution of cone photoreceptors ac ross the retina, determined using conventional light and fluorescence micro scopy also varies between the two species and may reflect differences in th eir visual ecology.