IN northern Europe, broad four-year oscillations in small rodent and raptor populations are synchronous over hundreds of square kilometers 1-6. Crashes in vole populations can induce wide emigration (> 1,000 km) of their predators 7 -9, but almost nothing is known about how predators rapidly detect areas of vole abundance. Here we report on laboratory and field experiments on voles (Microtus agrestis) and kestrels (Falco tinnunculus). Voles mark their runaways with urine and faeces, which are visible in ultraviolet light. Wild kestrels brought into captivity were able to detect vole scent marks in ultraviolet light but not in visible light. In the field, kestrels hunted preferentially near experimental nest-boxes where artificial trails were treated with vole urine and faeces. We suggest that kestrels flying over an area can see and use vole scent marks to assess vole numbers. This ability would enable kestrels to 'screen' large areas in a relatively short time. Our results provide a novel explanation for how raptors detect patches of high vole densities without prior knowledge of local food resources.