Kestrels hunt by hovering, holding the head almost motionless while the wings and tail compensate for wind. Sustaining that costs energy, so choosing where to hover matters more than hovering longer.
Work published in the 1990s, chiefly by Viitala and colleagues on Eurasian kestrels, showed that the birds preferred arenas marked with vole urine and faeces when ultraviolet light was available, and lost that preference when the ultraviolet component was removed. Vole scent marks reflect ultraviolet, and kestrel vision extends into that band, so the trails offer a visible map of where voles are active.
The finding is sometimes stretched into the claim that kestrels see urine trails glowing across a whole field from height. What the experiments support is a preference under controlled conditions; how strongly the cue guides hunting in real landscapes, where trails age and weather, is less well quantified.