You might be expecting a ‘Fowl Friday’ post, but this week we’ve decided do an ‘Owl Friday’ post instead.

Juvenile Great Horned Owl (Bubo virginianus) - © Curbstone Valley Farm

In our February Garden Update we were lamenting the fact that hoards of voles (Microtus californicus) have moved into our orchard.  As the orchard is fenced to keep out the deer, it also keeps most of the vole’s natural predators, like the coyotes and bobcats, from reaching them.  Hence, the vole population here has recently exploded.

Owl Lunch: Vole (Microtus californicus) under the gopher guard that protects our Spitzenburg apple tree

However, not all of the vole’s predators are encumbered by fences.  Especially owls.

Shortly after we moved here we demolished some rather crude and unsightly built-in-place bookshelves in the guest bedroom.  Even though these shelves were aesthetically bereft, they had been constructed from solid 3/4 inch wood. For the last few years we’ve stored the wood in our workshop, and there it sat, waiting to be re-purposed as part of another project.  What could be a more perfect use of recycled wood…than owl boxes!?!

Before building an owl box though, we had to know which owls to build a box for.   We know from recognizing their vocalizations that we have at least three species of owl that call Curbstone Valley home.  The Great Horned owl (Bubo virginianus) pictured above, Saw-Whet Owl (Aegolius acadicus), and Western Screech-Owl (Megascops kennicottii).

Western Screech-Owl (Image Source: Public Domain - US Fish & Wildlife Service)

It’s unlikely however that we have the ubiquitous Barn Owl (Tyto alba), due to our densely forested areas, which is unfortunate, as they supposedly relish voles the most.  However, the owls that do live here still enjoy a tasty rodent or three.

Great-Horned Owls don’t use nest boxes.  However, both the Saw-Whet and Screech-Owls naturally nest in tree cavities, and will readily use nest boxes if they’re available.

Northern Saw-Whet Owl (Image Source: Public Domain - US Fish and Wildlife Service)

A simple online search quickly turned up numerous Western Screech-Owl nest box plans.  After reviewing a number of them, we came up with a simple design for our box.

Saw-Whet Owls, and Screech-Owls have similar needs in regards to overall nest box dimensions.  We constructed this box specifically to suit our Screech-Owls, but either species may utilize it.  As construction projects here go, this owl box was tremendously easy to build.  In just a few short hours, with some simple straight cuts, a few wood screws, wood glue, a little silicon caulk, and some paint, a new move-in condition box was quickly put together.

Once all the pieces were cut, they were ready for assembly.

We began by cutting all the pieces to size (click image to enlarge)

The interior of the box is 8 inches square, and 15 inches high from the top of the floor, to the top of the face.  The most important consideration, other than overall dimension, was the size and location of the entrance hole.  For Western Screech-Owls an entrance hole 3 inches in diameter is recommended.  The distance from the top of the floor to the top of the entrance hole ideally should be 12 inches.

The entrance hole is most easily cut using a 3 inch diameter hole saw attached to a drill, however, our largest hole saw was only 2 inches, so we free-handed the cut with a jig-saw.

Shallow cuts were made on the interior of the front plate to improve traction for owls leaving the nest box

Shallow rungs were cut into the inside of the face board using a router.  Precision is not required, and as you can see we free-handed those cuts too.  The rungs are simply to help young owls gain traction when scrabbling up toward the entrance hole during feeding and fledging.

Note that the wood we were recycling was already painted on one side, so we turned the unfinished surfaces toward the interior of the box.

The side walls were attached to the back plate and the floor using wood glue and screws

We used wood screws for assembly, in part because this allowed us to make a simple swivel hinge for the door. Although most owl box plans we looked at suggested hinging the roof, we favored hinging the front of the box instead.  First, with the hinge on the roof, we were concerned about excess water leaching down inside the box.  By fixing the roof in place we were able to seal the seam where the roof meets the backboard with a little caulk, to help make it water-tight.  Second, we wanted the box to be easy to clean.  It seemed much easier to us to reach in from the front of the box, than down from above.

With the front plate hinged, minor debris can be cleared from the box without having to take it off the tree

In case water finds its way into the box, we nipped the corners off the bottom board, and drilled a central drainage hole.

The bottom board has drainage holes, and the face plate 'door' is secured with a spring latch

The face board is positioned to leave a small gap just under the roof for venting.

A gap at the top of the front plate helps to increase ventilation

With sufficient ventilation, and drainage, our box was almost complete.  Note however that we did NOT install a perch at the entrance to the box!  Whether you build or buy owl boxes, please ensure they do NOT have entrance perches.  Owls don’t need them, and they merely facilitate predators gaining entry to the box to steal eggs and young.

With our new owl box fully assembled, the next task was to prime and paint the exterior surfaces.  We didn’t have a choice in the wood species used for this box as we were recycling wood we already had on hand, and unpainted fir would be very short-lived here.  Had we built the box from redwood, or another rot-resistant species, we could have skipped the painting step altogether.

With a little paint though, we went from hideously ugly bookshelves, to a beautiful owl box, in less than a day…

The owl box was first primed, and then painted with an exterior semi-gloss paint to resist the weather

The next morning the box was bolted to a large Douglas Fir tree approximately 20 feet above the ground, with a lovely view of the meadow and orchard (and hopefully a clear view of our vole population).

"Wanted: Vole Control Supervisor in exchange for a move-in condition studio apartment, with excellent views, in a serene woodland setting"

As we still have a lot more wood left over, and a lot more trees on the property, we’ll definitely be building more boxes.  Our goal is to soon be finding a LOT more of these…

Owl castings (pellets) collected from below a Great Horned Owl nest on the farm (click to enlarge image - do you recognize any of these bones?)

We know that the owls alone won’t be able to keep the vole population below potentially damaging levels, but every vole they take, is one less we have to worry about!