They have ultra-violet vision!

Common Kestrel (Falco tinnunculus)
Common Kestrel (Falco tinnunculus)

A fact that I learnt today about this species today is that are able to see near ultraviolet light, allowing the birds to detect the urine trails around rodent burrows as they shine in an ultraviolet colour in the sunlight.

https://en.wikipedia.org/wiki/Common_Kestrel