Wednesday, August 29, 2007

Blakeman Responds:Voles, Owls, and Red-tails

The Milton Krider's Red-tail sits on her wooden power pole perch, watching for rodents.
As John Blakeman had put forth the hypothesis that the reason Barn Owls were becoming scarce was because of the diminishment of rodents due to row crops instead of pastureland in the Midwest. I asked, if that were the case, why were country Red-tails, who tend toward a diet high in small rodents also, doing well? Here is his response.


Good question. If there aren't enough voles and other rodents for barn owls today, why are there record numbers of red-tailed hawks, whose primary diet is the same "missing" rodents.

A partial answer involves the hunting techniques and habitat preferences of the two species. The barn owl is half the size of a red-tail and is not very powerful. Unlike the red-tail, it can't (and doesn't) take anything larger than a rat. The species is stuck with voles and mice.

The red-tail is large and powerful. If it wants to, it can take cottontail rabbits, muskrats, all local reptiles, and even grasshoppers. In the spring, red-tails occasionally take fish from local streams. Red-tails can eat virtually all animals less than five pounds in weight.

But in fact, the majority of the red-tail diet is voles and mice, the exact same diet of barn owls. Strength and prey preferences aren't the primary controlling factors. It's really habitat preferences. Red-tails spend lots of time sitting out on utility poles, looking down into the grassed ditches for ambling voles, mice, and rats.

Barn owls, however, prefer to hunt in low, grazed meadows, where the grass is kept low by sheep or cows, where the mice and voles are easily seen (or heard). Red-tails commonly drop through two or three feet of dense weeds or brush to take their prey. Barn owls are more reluctant to do this.

The prime factor limiting modern barn owl populations, however, is probably---can you believe this?---barns. Barn owls are cavity nesters, requiring large, enclosed spaces to nest (such as the ancient caves). Modern barns are closed, have no hay mows or other interior high shelves, or other spaces for a nest.

Here in Ohio, the Division of Wildlife has promoted the installation of large barn owl nest boxes in or on barns. The program has been very successful, but only where the new nest boxes are immediately adjacent to large grazed meadows and prairie restorations that can support hundreds of acres of vole habitat.

The limiting factor in modern barn owl populations is not so much the lack of prey and habitat (although the loss of grazed meadows is surely a factor), it's really the lack of nesting sites.

If barn owls are resurging in Iowa, it's surely because more nest sites are available, either from the opening of old barns (boards falling off), or more likely, because people have installed barn owl nest boxes in or on barns. Here is one barn owl nestbox design website:

--John Blakeman
And Beth Edwards of the U.K. sent in this link for Wirral's Barn Owl Webcam. Enjoy!
Donegal Browne

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