Sunday, August 27, 2006

Blakeman on voles


Bill Trankle’s close observations of a vole on his property are interesting, particularly because he got to watch a Cooper’s hawk devour the specimen. Cooper’s hawks prefer to take birds, but will occasionally take a small rodent.
Bill asks how many voles might a red-tail consume each day. A good question, as it relates to both the number of voles in a red-tail’s territory, and also to the number of red-tails that can be supported in a large, regional area.
Microtus pennsylvanicus, the common field vole, ranges across the northern portion of North America, from the southern tier of Canadian provinces to all but the southernmost tier of American states. It is absent from the far western states, but other similar small rodents provide red-tail provender out there.
The little creature averages about 70 grams, about 2 ½ ounces. In the summer, a red-tail needs from two to four of these each day to survive. In winter, three to five are required.
Although the following will contradict a great deal of textbook verbiage – none of which is supported by real field studies – I'll deflate a common conservation dictum related to voles and other small rodents. I'm treading on dangerous turf here, as this will contradict a great deal of conservation gospel. But my data, I believe, will support the deflation of the erroneous theory.
The theory is this. We need hawks flying around the landscape to control rodent populations. If we killed off all the hawks, rats, mice, and voles would explode in unchecked numbers. Hawks and owls are essential rodent predators, holding their populations in check. Hawks are “good” and rodents are “bad.”
A nice thought, one that was used in the passage of hawk protection laws. But the numbers don't add up. Here they are.
Studies have shown that typical vole populations average 8 to 10 per acre. During population highs, there can be as many as 50 to 60 of the little rodents in each acre.
Adult pairs of red-tails in rural areas typically have a territory or home range of about two square miles. In areas with reduced prey populations, territories can be larger. With lots of prey, territories can be as small as a half-square mile or so. Let’s use the two-square mile as a working number.
A square mile is 640 acres. Two square miles is 1280 acres. If each acre had only 8 voles, the territory would have 10,240 voles. If each of the pair of red-tails ate five a day (10 for the pair), the annual vole consumption would be 3650, only a bit more than a third.
Of course, the voles aren't just sitting there waiting to be eaten by a hawk. They are inveterate breeders, producing 5 to 10 litters each year, with 4 or 5 pups in each.
Anyone can see that a pair consuming 3600 voles in a territory each year just doesn't put a dent in the always-expansive vole population. Contrary to the well-meaning contentions of hawk protectionists, red-tails don't and can't “control” mouse or vole populations.
Red-tails should be (and are) protected for better purposes, because they are noble, natural denizens of North American wild areas – and now, even of places like Central Park -- not because they supposedly limit rodent populations.
And one last note. Unlike Bill’s Cooper’s hawk, a red-tail never takes 10 minutes to consume a vole. It’s often more like 20 seconds, usually in two or three powerful, flesh-ripping bites. The bird’s only concern is to get the captured specimen into the crop as fast as possible. Red-tails are never models of culinary decorum.
– John Blakeman