Hello,

This article may be of interest.

--Dan
Dan Victor <dvictor at u.washington.edu> Seattle, WA

Newgroups: americast.wpost

PUBLICATION DATE: 2/6/95

By Curt Suplee

If you're a European falcon, when you're out of voles, you're out of
business. The small, mouse-like rodents are so important to the falcon
diet that when vole populations drop off in Northern Europe, the birds
will migrate hundreds of miles in search of more. Survival demands the
ability to determine, rapidly and over a huge area, where the most voles
are.

Little is known about how the falcons do it. But now Finnish scientists
report in the Feb. 2 issue of Nature that the answer is ultraviolet
light. Voles mark their trails with urine and feces -- which, unlike the
surroundings, are visible in UV light. Thus a falcon aloft can disregard
everything but the UV excrement signals, which it is apparently equipped
to see.

The researchers tested this thesis two ways. In controlled indoor
studies, they offered the birds (Falco tinnunculus) a choice of four
places to hunt: UV-lit vole trails, a UV-lit area with no vole marks,
vole trails in visible light and a no-vole area in visible light.
Falcons preferred the UV vole-trail areas by a wide margin, which
suggests that they "were able to detect vole scent marks in ultraviolet
light, but not in visible light," the scientists found.

An experiment in the field gave the birds the choice of dozens of
places to hunt in an area of 15 square kilometers within which the
researchers had created artificial vole trails marked with vole urine
and feces, as well as unmarked artificial trails. The falcons
overwhelmingly chose the marked areas, suggesting that "they were not
using the trails themselves as cues for hunting."

It is unknown whether the European falcon has eyes that are sensitive
to the UV range. However, the researchers note, "most other diurnal
birds studied so far have proved to be sensitive."