A succession of mild winters
has allowed rodent populations to build up in the Pacific Northwest. Of particular concern to growers are meadow mice, also called voles. They belong to the family Microtinae. Although their populations are cyclical, and will eventually drop, that is little comfort to a grower.
In the field, voles build subterranean nests. During the breeding season they are dispersed. Breeding is continuous from early spring through late autumn. We have found nests with hairless young from March though late November.
Voles create subterranean food caches which resemble silage. They gather and cache orders of magnitude more plant material than they need to stay alive, which is why they are such a problem in the field. The caches include leaves and roots. Their manic foraging takes place in a 50-foot or so radius, and happens almost continuously. As foraging supplies become limited, the animals broaden their food selection.
Control measures
We do not use poisons. We have tried traps of various designs suggested to us, and we admire the amount of effort people expend to ensure trapping success. Traps don’t work for us. We do dig out the nests and kill the inhabitants when time and opportunity permit. We have certainly had a bigger impact on the population this way than with traps.
In our experience, the most effective control is accomplished by increasing predator populations. The two predators most easily accommodated on a farm are the kestrel, the smallest of the falcons, and the barn owl. These two birds of prey are found throughout the United States. Screech owls can also be helpful, though they prefer a wooded habitat to the open fields we need patrolled. As our orchards mature, we will give greater consideration to screech owls.
Mice and voles are the preferred prey of kestrels and barn owls. The two species typically do not take songbirds or bats. Screech owls will feed on small bats which should be a consideration when providing nest sites. If you have invested time and effort in developing roosting structures for bats, you may want to pass on a hungry family of screech owls.
The populations of the barn owl and kestrel are often limited by the availability of appropriate and safe nesting sites, and they readily accept artificial nesting structures to ameliorate the situation. We have increased the populations of both birds by increasing nesting sites on the farm. Our goal is to have the populations of the two birds determined by prey populations, not nest site availability.
Nest boxes for barn owls
Our barn owl “boxes” are made from used fish emulsion or kelp extract barrels, typically 30 or 50 gallons, as shown at right. We construct a wooden landing platform to make the exit and entry comfortable for the bird, and cover the bottom with bark dust or dry oak leaves. Never use cedar chips for bird boxes. The web site www.rain.org/~sals/barnowl.html provides many nest box designs that can be made out of scraps on hand.
Our boxes are located inside our open barns and we try to place them 15 feet or higher above the ground. Barn owls are extremely high-strung and do not win any points when it comes to intelligence. Their optical nerves occupy most of the cranial cavity, leaving little room for the fine points of life and no room for humor.
The boxes can be placed in trees. We have put out some nest boxes at the fringe of our woodland, but the barn owls favor those under cover.
Barn owls hide themselves well inside the box and the only clue that the hen is incubating may be a fresh pellet or two in the barn, or a downy feather clinging to the opening. The male will feed her during incubation, which makes a pair of boxes useful so that he will have some place to hide, especially when the nestlings start to fill the box.
Even if barn owls currently nest in the barn, boxes will make their presence more manageable if you are constantly flushing them during daylight hours. They will abandon eggs or even young if disturbed too often. We can move machinery and forklifts under the nesting boxes without flushing the birds. As long as they have a place to hide, they will acclimate to the activity of the barn.
Last year, a nestling fell out of the box and crouched in an aggressive posture on the ground. After a call to the Audubon Society to make sure we would cause no problems, we hauled out a ladder and heavy gloves and stuffed the bird back in the box. All the while, the mother was deep in the box hissing fiercely. The young owl grew up without further incident, though Anthony has never been quite the same.
The barn owl has a long wingspan that is best suited to gliding silently over open fields, and is poorly adapted to maneuvering in woodlands or among barn rafters. During the training phase, the young fly awkwardly around the barn, crashing into walls and rafters. The parents bring in rodents and drop them below the owlets. The floor of the barn is littered with voles that have died from the training ordeal. The vocalizations of the barn owl are hair-raising, bordering on the grotesque. The most interesting is a rapid clicking sound which we think is used to flush rodents.
Kestrels
Kestrels use boxes with a 10x10- inch floor and a 4-inch opening about 10 inches above the floor. We generally cut a U-shaped opening at the top of the box, rather than a round hole. We use pine planks and paint the outside for longer life.
The boxes should be placed 15 feet or higher above the ground. They may be placed on the sides of barns near the peak of the roof, attached to tall poles, or on dead trees or snags. Kestrels will stand guard over the nest box from a nearby tree or power line, so have the box placed where the view and access is unobstructed by tree limbs. Kestrels and starlings will scrap over who has control of the box, but if it is in the open, the more powerful kestrels rule.
Kestrels have one or three young. The chicks often fling themselves out of the nest well before they can fly. A bit of shrubby cover that can provide protection to the young during this vulnerable flightless period is a consideration in siting the box. The normally noisy kestrel parents become silent and invisible while the chicks are on the ground.
We have four or more pairs in the eight boxes around the property, as well a couple of pairs in natural cavities. If we relied solely on natural cavities, our kestrel population would be limited to those two pairs.
Screech Owls
Screech owls use a box with the same dimensions as the kestrel. They are placed at the edge of a woodland or in an orchard with the opening facing northward. Among the vocalizations of a screech owl is a beautiful warbling song, completely unlike any other owl.
Management considerations
We keep track of occupants in all of our nest boxes. If one fails to attract birds, we move it. We have observed kestrels using the same natural cavity successfully for at least eight years, so we are inclined to regard box cleaning as unnecessary busy work. If birds stops using a box, a quick autumn inspection is warranted. The owls occupy the box all year and it needs no cleaning. In fact, a thick mat of urine and pellets is thought to be beneficial. Be sure to put the box in a convenient place, as the owls can live 30 years.
Removing vegetative cover exposes voles and other rodents to predators, especially on foraging runs. However, on our farm we have long-tailed weasels which need cover to hunt, or they will fall prey to the larger hawks, so we regard aggressive cover removal on the fringes of fields as counterproductive. Moreover, removing non-crop cover along the field fringe when the voles are present may shift the rodents into the crops. In the fields, we do try to minimize cover as weasels do not hunt in the open.
Watching the behavior of the raptors is useful. Red-tailed hawks and kestrels prefer to hunt from perches, especially in cold weather when there are no chicks to feed. Typically they work from power lines or poles. They will also sit in small trees or on fence posts. Owls also hunt from perches. Observing this behavior, we provide perches where colonies of voles are working. A simple T-post pushed in the ground does the trick, especially for kestrels, though we also add a short crosspiece on our luxury models. A crossbar perch is more comfortable for larger raptors. These perches can be moved around the field with little effort.
It is an easy matter to increase the nesting populations of kestrels and barn owls, and they are good predators to have on the farm. They are fun to watch and give the farmer a pleasant diversion to write about in the market e-mail. We find it more gratifying to see owl pellets with several vole skulls plastered in the furry matrix, than checking and resetting traps daily.
Anthony and Carol Boutard operate Ayers Creek Farm, an Oregon Tilth certified organic farm in Gaston, Oregon. They grow canefruits (red, purple and black raspberries, blackberries, loganberries and boysenberries), plums, chestnuts, winter vegetables, fresh shell and dry beans, and specialty grains. They may be contacted by email at [email protected].
