The visual ecology of avian photoreceptors

The spectral sensitivities of avian retinal photoreceptors are examined wit h respect to microspectrophotometric measurements of single cells, spectrop hotometric measurements of extracted or in vitro regenerated visual pigment s, and molecular genetic analyses of visual pigment opsin protein sequences . Bird species from diverse orders are compared in relation to their evolut ion, their habitats and the multiplicity of visual tasks they must perform. Birds have five different types of visual pigment and seven different type s of photo receptor-rods, double (uneven twin) cones and four types of sing le cone. The spectral locations of the wavelengths of maximum absorbance (l ambda (max)) of the different visual pigments, and the spectral transmittan ce characteristics of the intraocular spectral filters (cone oil droplets) that also determine photoreceptor spectral sensitivity, vary according to b oth habitat and phylogenetic relatedness. The primary influence on avian re tinal design appears to be the range of wavelengths available for vision, r egardless of whether that range is determined by the spectral distribution of the natural illumination or the spectral transmittance of the ocular med ia (cornea, aqueous humour, lens, vitreous humour). Nevertheless, other var iations in spectral sensitivity exist that reflect the variability and comp lexity of avian visual ecology. (C) 2001 Elsevier Science Ltd. All rights r eserved.