This is not a great picture, as I was far away, shooting into the sun without tripod or brace, and it was 0 degrees with wind, but... an American Kestrel.
The American Kestrel or Sparrow Hawk, sometimes called a "spar", is actually a small falcon — Falco sparverius.
American Kestrels have cones in their eyes that allow them to see in the ultraviolet spectrum.
Why is that useful? Simple: mouse urine glows purple in ultraviolet light — one reason "black light" is used to find old pet urine stains on wooden floors.
When a Sparrow Hawk hunts along a hedge, it is able to look for urine stains that mark mouse holes, runways, and nests.
Mice and rats mark their burrows and runways with urine scent trails because, like most mammals, they do not have very good eyesight. Scent cues help rats and mice orient themselves in the complex world of hedge, forest, field and barn. The result is that a rat or mouse can "run" a scent trail very fast without spending time in the open trying to figure things out.
Experienced human ratters know this about rodents, and so they will drag their boot across the middle of a shed and spin long boards perpendicular to where they have been lying, all in an attempt to break up or disturb invisible scent trails. A few seconds hesitation by a confused and disoriented rat is just the edge a terrier needs to even the odds on the farm.
For a Sparrow Hawk or Kestrel, visible urine trails enable it to focus on areas where small rodent activity is heaviest. By simply flying over a hedge or field, and then lighting on a fence post, telephone pole, or dead tree near visible urine marks, a sparrow hawk can dramatically increase its chance of finding lunch.
Once activity in the grass is spotted, a sparrow hawk will face into the wind and hover low over a grassy area -- a tactic that earned it the name "windhover" or "wind fucker" in 17th Century Holland, when the terminology was considered a little less offensive that it is today.
As for ultraviolet vision, it's not all about food -- it's about sex too. It seems that all those rather drab-looking birds we see in the hedge look a lot prettier when they are seen through the lens of an ultraviolet-sensitive eye.
Birds species in which male and females look very much alike to us look very different to the birds themselves, as they have twice as many cones in their eyes and so can see shade and colors we miss altogether.
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