Building for Wildlife: Nest Boxes

by: Bob & May Bancroft

Looking for a wildlife project?

When a neighbour cut down a hardwood several winters ago, it hit the ground and split open where a hole had weakened the tree. Thirty-five flying squirrels, one red squirrel and a bat spilled groggily out of the maple. This squirrel "condo" may have been the only one around. Some very interesting wild animals search out trees with holes for nesting, resting and escaping predators or winter weather.

Where cavity trees are scarce, nest boxes are useful alternatives. Helping wildlife can pay you dividends in more than one way. The busy flickers which adopted one of our boxes last year eat as many as 5000 ant eggs, larva and ants in one meal. thanks to their presence, our house did not suffer the usual spring ant invasion. If you live near water, Nova Scotia has four species of ducks that nest in trees. Some have resorted to chimneys when a suitable tree could not be found. This is not the way to have roast duck! Other nest box users eat their weight each day in mosquitoes and other flying insects.

Bird houses, or nest boxes, can be fancy or simple. This article discusses simple designs. materials need not be expensive. We use leftovers: ends of pine boards, plywood pieces, old barn boards, rough sawn lumber, and other scraps. Slabwood leftovers from a lumber mill will do nicely. One can even consider a section of hollow log found in the firewood pile.

The diagram in this article offers one design. Fancier offerings exist for waterfowl, bluebirds, owls and purple martins. Interior sizes vary depending on the bird species as shown in the chart.

Keep the following construction details in mind:

  • Make the entrance hole the correct size for the kind of bird you want.
  • The top of the entrance hole should be about 3 cm (1 1/4 in.) down from the underside of the roof, in most cases.
  • When using lumber of different thicknesses, keep in mind that inside dimensions are the most important to birds.
  • Do not attach a perch outside the nest hole. A perch helps nest raiders attack occupants.
  • Drill a few holes in the floor of the house to allow water to drain.
  • Drill some small holes on the sides near the top of the box, below the roof overhang. This provides ventilation from heat, and a little light.
  • Stain or paint only the outside of the box. If you use paint, lighter colours or white reflect more light and minimize summer heat in open sites.
  • There should be no protrusions (nails, screws) inside the box that could cause injury.
  • The entrance board should be rough on the inside so young birds can climb up to reach the hole. Wire mesh can be stapled to the inside, or a series of shallow, horizontal saw cuts made across it.
  • The top (or one side of the box) should be openable, for a clean-out after the nesting season. Pre-drill holes in the closed position; then press in nails that can be pulled out later with fingers for the house cleaning.
  • A groove across the underside of the roof overhand (on the front and sides) will act as a rain drip.
  • Most birds bring their own nest material. An empty nest box is fine for them, but flickers, woodpeckers, owls and ducks prefer a layer of wood chips or shavings (not sawdust) to a depth of 2.5-7.5 cm (1-3 in.)in the bottom of the box.

Attaching the box
Nest boxes can be attached to trees, poles, posts or to the side of a house. You can do this by:

  • Extending the back wall of the bird house above or below the rest of the box. Nailing or drilling and screwing can be easily done on the extension piece.
  • Attaching another longer piece of wood to the outside of the back wall.
  • Hanging the box against a tree trunk or from a limb by means of a hook or hooks screwed into the top or sides of the box and the tree.
  • Wiring the box from eyelets, nails or screws on the box to nails or screws in the tree or post. Do not wrap wire around a living tree. It could injure the tree as it grows.

Read about the birds you want to attract. The chart will provide information so you can place the nest box at a good height on existing trees or installed poles. In some cases nesting territories of a species are given, so you can estimate the number of boxes to erect in a given area. Place boxes in quiet areas where human traffic is at a minimum.

Position the entrance hole so it receives some sun, but not so much that the residents overheat! If possible turn it away from the usual direction of rain and strong winds.

Clothesline poles can be suitable if the box is located away from or above flapping clothes.

Tree swallows prefer open areas within sight of water; nest boxes can be placed as close as 10 feet apart.

Chickadees like to nest near bushes or shrubs.

Purple martin colonies will probably not be used unless these birds already frequent the area. Obtain more construction information before you build a "condo" for them.

Wrens need two boxes, one for the nest, another to store nesting material.

Flickers adopt a nest box more readily if it is placed above surrounding foliage.

Wood ducks, goldeneyes, hooded and common mergansers prefer a nest box near, or standing in, water. Poles for nest boxes in water are easier to erect by chopping through ice during the winter.

Starlings and house sparrows are imports and early nesters that often take over nest boxes intended for native birds. If you like them, keep them! If they become a problem, cover the entrance holes of the boxes until your desired species show up in the spring.

Red and flying squirrels will think you put up the boxes for them. Perhaps you did. Alternatively, put up more boxes on solitary trees that cannot be reached by jumping or gliding from another tree. Place a guard around the trunk of the tree some distance up so jumping from the ground is impossible. The guard can be an inverted cone of metal or plastic, or a 0.6 m (two-foot) strip of metal or plastic wrapped around the tree.

Raccoons may require a one metre (three-foot) strip, or a nest box of their own. These opportunists can be a problem with waterfowl nesting boxes. An oval rather than round entrance hole, and a box with proper depth may curtail their egg-eating tendencies.

Checking your Box
The early stages of nesting, when eggs are being incubated, are critical times when the box should not be checked. Later inspections should be done quickly, when the adults are away feeding. Dead nestlings should be removed. They attract extra insects that make life tough for the nestlings that remain.

Old nest materials, wood chips, and shavings should be cleaned out after the young leave the nest. this keeps down parasites that "welcome"the next group of nestlings. Some species nest more than once a year, so a close watch must be kept for the first batch of new fliers. Extra boxes put up close-by are an alternative, and may attract double-nesters out of the first nest.

Some nest box users can be quite secretive. The flickers at the back of the orchard fooled us until the young began clamouring to be fed at dawn. One of our swallow boxes last year hosted a colony of wild bees. There was no pollination problem in our garden!

For more details on barred owl, wood duck, bluebird, and purple martin houses write to the Department of Lands and Forestry at P.O. Box 68, Truro, NS, B2N 5B8.

Bats are our only defence against night-time biting insects, which include hordes of mosquitoes. In the next issue we'll explain how to build a bat-room.