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Why can many birds and fish detect ultraviolet light?

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details of the mammalian eye — Photoreception The mammalian eye has a cornea and a lens and functions as a dioptric system, in which light rays are refracted to focus on the retina.

Many birds and fish possess the ability to detect ultraviolet (UV) light because they have evolved specialized photoreceptors that are sensitive to these shorter wavelengths. While humans typically have three classes of cones for colour vision, many birds and fish have four or even five types of cone cells, which include specific receptors for the ultraviolet spectrum.12

This capability is facilitated by the presence of specific opsin proteins within their photoreceptors. By varying the amino acid composition of these opsins, these animals can "tune" their visual pigments to respond to different ranges of light, including UV. This expanded visual range allows these animals to perceive environmental details, such as patterns or signals, that are invisible to humans. For example, some animals use UV-reflective markings for communication, which appear as distinct signals to those capable of detecting them. Additionally, some birds and reptiles further refine their colour vision through the use of coloured oil droplets in their eyes, which modify the spectrum of light reaching the photoreceptors.1234