Natural predators come in all sizes, shapes and classes. There are generalists that eat whatever they can get a hold of to specialists that just hunt and consume certain species in a particular way. Lions and cheetahs are good examples. Here in North America, owls own the night skies. Great horned owls (Bubo virginianus) eat a list of mammals, birds, and reptiles that is reminiscent of what the cartoon Tasmanian devil might eat (using about the same table manners).
Great horned owls have been known to eat young raccoons, red-tailed hawks and even great blue herons. They will consume pounds of flesh and not eat for nights due to a slow metabolism. This species is our only representative of the booted eagle owl clan, a worldwide distributed genus of owl that contains among its ranks the world’s largest species of owls. These birds approach 20 pounds and have the physique of a trash can.
Barn Owls for Rodent Control
Barn owls (Tyto alba) are quite different from the eagle owls, focusing on rodents almost exclusively and especially rats, mice and gophers. They also eat moles, voles, shrews and anything brown and fuzzy that runs around on the ground at night. They wash this all down with the occasional cricket and wayward beetle.
Barn owls are "tachy metobolic" and eat roughly half their own weight each night in crop-eating rodents. One biologist's study showed that one pair of nesting California barn owls can consume as many as 2,000 rodents per year when feeding their young. I reckon that number to being about a heaping long-bed pickup load's worth. Their young eat so many rodents in such a short time that they weigh as much as their parents in about 50 nights and fledge in about 65 to 70 nights.
All extant raptors have the steepest growth curves of any known vertebrate. Their cousins the tyrannosaurs had the steepest growth curves ever calculated: from a chicken-sized hatchling to a 40-foot long, nine-ton giant in 5-7 years. Some paleontologists believe that this gargantuan appetite precluded a parental situation, theorizing that they had to be pack animals!
Pause on that hunting scene for a moment. In the here and now, it is quite easy to attract barn owls in most states. A properly placed and well configured barn owl nesting box, if left alone long enough, may attract a nesting pair.
Siting Barn Owl Nesting Boxes
Good places to install nesting boxes are in trees and on galvanized metal poles. In trees, I like to hang them by a particular chain called "linked loop." A North-ish (from northwest to northeast) exposure is great, east is OK, South is good, West, though? Almost never. The hot afternoon summer sun will often radiate the inside of the box, making it too hot and forcing the owls out to go roost and nest somewhere else.
The point of the compass method is part of the placement decision with the lay of the land factored in. An open flight path is needed for an owl's winged approach. If the approach lane has a totally open view of, say, a lawn, then great. They like a "commanding view" as biologists say.
For me, a simple depth-of-field view from the backyard tree-mounted nest box doorway facing down a driveway with a view of a street cul de sac has worked in the past. It was the only lengthy view there was at this property. I imagined the owls spying rats running across the open pavement out front foraging for food and the owls appreciating that view. That client got owls in two weeks. My record is five hours!
Building Barn Owl Nesting Boxes
A good configuration but uncomplicated is a design I offer called the "basic box. It is made of ½-inch plywood and painted a camouflaged green color.
Its cut dimensions are:
• Bottom: 12 inches wide by 23 inches long
• Sides: 12 inches wide by 14.75 inches high
• Front and back panels: 14.75 inches high and 24 inches long
• The lid or roof: 16 inches wide by 24 inches long
Glue and screw the box together or use nails or heavy narrow crown staples. For all nest boxes: Place the box upside down. Using a paintbrush, paint the ceiling (for now the floor) with a slurry of fireplace ashes and water. Allow time to dry. This helps prevent bees from colonizing the box.
The doorway is critical, starting one inch from the left side on the face panel, 14.75 by 24 inches long, and 1 inch up from the deck, cut out a bathtub-shaped doorway 8.5 inches high and 4.5 inches wide. The bottoms of some smaller antifreeze or brake fluid jugs work passively to sketch a pattern with a pencil. This shape and being close to the deck allows the female owl to clean out the box, eliminating the need for humans to do so. This lower doorway allows the young to jump out of the box in heat waves instead of perishing trapped inside. The chicks are naturally afraid of the doorway until older otherwise.
Next, drill two holes on each side 2 inches from the sides — four total — and 2 inches from the top near each corner. With two small bolts, add a 24 inches long stick, 2-by-2-foot lumber scrap, dowel or even a cut off broomstick for a perch near the door along the bottom of the box, and bolt it on solid.
Now sand the edges of the box smooth, top to bottom, and paint it. After it dries, run 6-foot-long lengths of ¾-inch linked loop chain through both holes, one then the other, to mid-chain length. End the chain with a "quick link" locking carabineer or some similar clip. I recommend you have an arborist (like me) install the nest box in a tree. Don't risk climbing trees using a trial-and-error siting approach. It’s not worth it. You could get hurt (or worse).
Mounting a Barn Owl Box
Tree-mounted. See video above of my tree-mounted box. You will end up running two chains through four holes put in the box, two on each side and wrapped around the branch in a "timber hitch" knot, completed with a level to assist in box placement aesthetics as seen in my video How to Install an Owl Box.
If you have no suitable level, stout branches at 15 inches to 45 feet high facing north-ish, east or south, then a pole-mount box might be the only option. These are jumbo-sized —2 3/8ths, 16 gauge — galvanized chain-link fence to rail tubes cut at 16-foot lengths. The box bolted offset to the backside of the box.
Pole-mounted. After the nest box is complete, drill two 5/8-inch holes in the end of the pole, one about 2 inches from the end cut and another about 9 inches down from there. Lay the box on the ground face down. Line up the pole on the back of the box and using the pole holes as a marker, drill two holes in the back panel of the box going through the pole holes first as a pattern. Run two same-size carriage bolts through both pole and box with the bolt head inside and washer and nut outside the pole.
Good spots for pole mounting are facing open ground, just east of a tree about 10 feet away to glean some summer shade. But not too close to a tree. Keep it conspicuous. If no shade, glue a second layer of plywood roof panel and keep your fingers crossed. They amazingly nest successfully in Borrego Springs of San Diego County. This desert town is surrounded by California's largest state park. It is “as hot as the anti-chamber of hell", as one British general commented when he chase the American “scoundrels" through South Carolina's Low Country during the revolutionary war.
Dig a 3-inch deep hole in the ground at a "owly" spot, and slide the pole in, packing dirt with the shovel and using a level as you pack in the dirt with the handle end of the shovel. Or you can take a schedule 40, 5-foot-long water pipe, 1.5 OD, and jack hammer it in the ground leaving about 16 inches above the soil surface. Then sleeve the 16-footer with the box at the top over the pipe. Drill a hole through all four walls of the pole and pipe about 12 inches off of the ground, and put a bolt through them all as a set pin so the pole and box will not spin.
If you live in a lightning-prone area, perhaps you may want to go the 4-by-4 or a series of 2-by-4 foot wooden pole. Be sure to put a metal flashing collar about the 4-by-4 and about 3.5 feet up to keep raccoons off the pole. It is best to have a few boxes hung up in different spots — a two box minimum, his and hers.
Tom Stephan works in the green industry treating sick trees to improve their vigor and vitality through anti compaction and soil fertility. He is a former certified arborist, a master falconer, and has incurable minimalist tendencies. Connect with him at Barn Owl Boxes, and read all of Tom’s MOTHER EARTH NEWS blog posts here.
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