Even with their superb visual acuity and excellent colour sense, extending well into the ultraviolet, kestrels find it hard to spot the drab voles which are their favourite food. Happily for the kestrels, however, voles communicate by leaving trails of urine—indeed, they pee almost continuously—and mole urine reflects ultraviolet light. For kestrels, hunting voles is simply a matter of following the arrows.
I must admit, I was initially irked by Ing's use of the word happily to describe what appeared clearly to be a marvellous hunting adaptation evolved by the kestrel. But not so: all birds can see into ultraviolet wavelengths, apparently; so kestrels can't have evolved their ultraviolet vision specifically to hunt voles. The apparent adaptation turns out to be a lucky coincidence, which the kestrel has put to good use—possibly refining it over time.
Ultraviolet mole piss detection isn't so much an adaptation as an exaptation, it would seem.