BECAUSE kestrels, as they hover above open land, can see in the ultraviolet light range, they are apparently able to spot the highways of voles and other small mammals and swoop on their prey, according to a Finnish study.

The birds can see the trails from far overhead because the voles, like other mammals such as mice, dogs and wolves, mark their trails and territory with urine or feces. The waste material is not only tagged with the odor of their species but also marks their highways because it absorbs ultraviolet light.

Many raptors, such as falcons and sparrow hawks, the American counterpart of the kestrel, are believed to have ultraviolet vision and, in treeless regions, this could be an aid to hunting.

In Scandinavia and other northern regions the populations of small mammals often oscillate in four-year cycles between overabundance and crashes. In a crash it becomes hard for raptors, such as kestrels and snowy owls, to find their prey. If, however, they can see a highway map of their prey's movements, they can rapidly scout large areas.