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sorry Richprins .. I'm not a specialist in bird's vision ( now probably..
) ... and I agree your response.
... according my researchs on web sites ...
The retina of the eye contains cones, which, when stimulated by different wavelengths of light, transmit colour information to the brain. In a human, there are three types of cones, enabling us to perceive three primary colours: red, green and blue. This is known as trichromatic vision. The combination of these colours enables us to perceive thousands of different colours.
Birds have a fourth cone, which is sensitive to UV light, and can perceive four primary colours, the additional colour being UV. This is known as tetrachromatic vision.
The retinas of most birds have four different classes of cones, rather than the three we have in our retinas. Birds have visual pigments maximally sensitive in the red, green and blue parts of the spectrum, like us, plus an additional pigment that is most sensitive in either the violet (400-426 nm) or the ultraviolet (355-380 nm). Note that although the peak sensitivities differ between the two forms of the fourth pigment – called VS and UVS, respectively – they both permit birds to detect ultraviolet light. A number of experimental studies have now shown that birds use their ability to see UV in much the same way as they use other parts of the visual spectrum, i.e., for finding prey and for signaling to other members of their species. Birds are often more brightly coloured in the UV than they are in our visual range.
The wavelength range for UV light is 3,000 to 4,000 angstroms or 300 to 400 nanometers, and the peak sensitivity for vision in most birds is between 3,600 and 3,800 angstroms or 360 to 380 nanometers. Our human range of sensitivity comes in somewhere around 4,000 to 7,000 angstroms or 400 to 700 nanometers. Our cornea and lens absorb UV light because of its potential to damage our
retinal pigments, so we humans cannot detect it. Birds, however not only have UV-transparent corneas and lenses, but also special retinal cone receptors with visual pigments that mostly absorb light in both the violet and UV ranges.
.. for the Kestrels ... no problem ...
Scent marks of mice are known to be visible in UV light but a new finding was that vole scent marks have also a special hue only visible in UV light. Therefore, predators that are able to detect UV could use scent marks to find vole patches and assess prey densities. Kestrels may be able to distinguish these differences. Other diurnal birds of prey, such as rough-legged buzzards, are attracted to vole scent marks.
.. some studies about the kestrels ...
here
and here
.. I have proposed "Owls" ... according my first reference found ... and I supposed that they were also able to use the same way to hunt or to recognize other birds, predators, obstacles ...
So, I accept your remark ...
... untill I find a study about the owls and UV vision ...
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