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, found that kestrels preferred arenas marked with vole urine and faeces when ultraviolet light was available, and lost that preference when the ultraviolet component was removed. The result was widely retold as kestrels seeing glowing urine trails across a field.
That retelling has not survived scrutiny. Raptors, kestrels included, carry violet-sensitive rather than ultraviolet-sensitive cone pigments, with peak sensitivity near 400 nanometres, and later reflectance measurements found vole scent marks no more ultraviolet-bright than the surrounding vegetation. The original arena preference stands as a finding; the mechanism offered to explain it does not, and no field study has established that kestrels track scent marks visually while hunting.