Habitat

Winter Roost Boxes

A roost box is a nest box turned upside down in almost every respect: entrance at the bottom, perches inside, no ventilation, thicker walls. Whether birds will use yours is genuinely uncertain.

A small songbird on a January night is running a survival calculation. It has burned the day's calories, it has perhaps fourteen hours of darkness ahead, and where it sleeps changes how much of its fat reserve it spends. A bird tucked into a sheltered cavity out of the wind, sharing its body heat with others, uses measurably less energy than one clinging to a branch in the open.

That is the argument for a roost box: an insulated, draught-free cavity in a yard that has lost its dead trees. It is a reasonable argument, and it is also the argument for leaving the dead trees alone in the first place, which is cheaper and works better.

This page covers how a roost box is built, where it goes, which birds use them, and what the evidence actually supports. Some of that evidence is thinner than the confident plans you will find elsewhere suggest, and this page says so where that is true.

Roost box versus nest box: the differences that matter#

A roost box is not a nest box you use in winter. Almost every design decision is reversed, because the two boxes solve opposite problems. A nest box has to stay cool and dry through summer and let a female see out; a roost box has to trap heat and block wind on the coldest night of the year.

If you already have and maintain a nest box, the fastest route to a roost is not a second box. It is to leave the nest box up through the winter, which several cavity-roosting species use readily. That, and the rest of the nest box argument, is covered in nest boxes.

Dimensions that work#

There is no single official plan, and published dimensions vary. These are the numbers most designs converge on for a general-purpose songbird roost box.

• Interior floor: 8 by 8 in (20 by 20 cm) at minimum, up to 10 by 10 in (25 by 25 cm).
• Interior height: 20 to 24 in (50 to 60 cm), which is what gives a stack of perches above the entrance.
• Entrance hole: 1.5 in (3.8 cm) round, or a 1.5 by 2.5 in (3.8 by 6.4 cm) slot, centered 4 to 6 in (10 to 15 cm) above the floor.
• Walls: 3/4 to 1 in (19 to 25 mm) untreated rough-sawn cedar, pine or exterior plywood. Never treated lumber, never metal, never a thin plastic shell.
• Roof: sloped, with a 3 to 4 in (8 to 10 cm) overhang on the entrance side to keep rain and snow out of the hole.
• Perches: three to five dowels 3/8 to 1/2 in (10 to 13 mm) in diameter, set at different heights and staggered from side to side so a bird is never directly under another one.
• Interior climbing surface: leave the wood rough below the entrance, or fasten a strip of plastic mesh, so birds can climb up and out.
• Access: one hinged side or a lift-off roof, so you can clean it and get a hand inside.
• Floor drainage: two or three small holes in the floor corners. This is the one concession to airflow and it is worth making, because wind-driven rain will get in.

For a larger box aimed at screech owls or kestrels, the design is different again, and it is really a nest box rather than a roost box. Owls use standard top-entrance nest boxes for daytime roosting as well as nesting, so build the nest box rather than a scaled-up roost box with a hole at the bottom.

Siting and height#

Placement matters more for a roost box than for a nest box, because the value of the box is entirely about shelter from wind.

1. Mount it 8 to 12 ft (2.4 to 3.7 m) above the ground. Higher is generally safer from ground predators and harder for you to service.
2. Face the entrance south or southeast in most of North America, away from prevailing winter wind and toward afternoon sun.
3. Choose a sheltered aspect: the lee side of a building, a trunk with dense cover behind it, or inside a group of evergreens.
4. Put it within a short flight of cover and, ideally, within sight of your feeders, so birds already using the yard find it.
5. Fix it firmly. A box that swings or rattles in wind will not be used.
6. If it is on a pole, fit a predator guard. The same reasoning as in baffles applies, and raccoons are the specific problem: a box full of sleeping birds is a serious prize.

Which birds actually use them#

Cavity roosting is common in winter, but the species differ in whether they do it alone or in company, and that distinction decides what a communal box is worth.

• Eastern bluebirds are the classic communal roosters, and groups packed into a single box are well documented. If you have bluebirds wintering, this is your best candidate. See attracting bluebirds and Eastern bluebird.
• Pygmy nuthatches roost communally in large numbers in cavities in the West, which is one of the better documented cases of huddling in North American birds.
• Carolina wrens roost in odd sheltered cavities, sometimes in pairs or small groups, and take readily to boxes, hanging plants and porch shelves. See Carolina wren.
• Chickadees and titmice use cavities every night, but generally roost singly in separate holes. A communal box is less obviously useful to them, though individuals will use one.
• White-breasted and red-breasted nuthatches roost in cavities and will use boxes, usually alone or in twos.
• Downy and hairy woodpeckers excavate or claim their own roost cavity and sleep alone in it. A box may be adopted by one bird, not a group. See downy woodpecker.
• Screech owls use nest boxes as day roosts year round, which is a good reason to put up an owl box rather than a songbird roost box if owls are what you want.
• House sparrows and European starlings will happily take a roost box, which in some yards is the outcome you get.

The honest evidence on uptake#

Here is the part usually left out. Roost box use is poorly documented compared with nest box use, and for a mundane reason: nesting happens in daylight over weeks and is monitored by large volunteer programs, while roosting happens after dark and almost nobody checks.

What that means for you is that the confident claims are thinner than they sound. Boxes going unused for one or more winters are a common experience, and no strong general figure exists for how often a roost box gets adopted. The variables that plausibly matter are whether your area already has natural cavities and dense conifers, whether cavity-roosting species are present in winter, and whether the box is well sited.

None of that is a reason not to build one. It is a reason to build one cheaply, put it somewhere sheltered, and treat any use as a bonus rather than a return on investment. If you want to know whether yours is being used, look for droppings on the floor when you clean it in spring, or watch the entrance in the last twenty minutes of daylight, which is when birds go in.

Cheaper things that work better#

In most yards, three habitat features beat a roost box, cost nothing, and benefit far more species.

• Leave standing dead wood. A snag with old woodpecker holes is a purpose-built roost that comes with insects attached. Where safety allows it, this is the highest-value thing on the list, and snags and dead wood covers how to keep one safely.
• Keep or plant dense evergreens. Spruce, cedar, juniper, hemlock and pine are where most small birds actually sleep in winter, and they shelter species that never enter a cavity. Regional choices are in native plants.
• Build a brush pile. Prunings stacked coarse at the bottom and fine on top give wrens, sparrows, juncos and towhees a windproof place to sit out a storm, at zero cost. See brush piles.
• Leave your nest boxes up, uncleaned or lightly cleaned, through the winter. Cavity roosters use them, and it is the one change that requires no work at all.

Set that against the calories a feeder supplies and the picture is clearer: food is one half of winter survival and shelter is the other, which is the argument running through winter feeding and, more sharply, extreme cold feeding.

Maintenance and the spring switch#

Put the box up in late fall, before the first hard freeze, so it is in place and familiar when birds need it. Leave it through the winter and check it in early spring.

Cleaning matters more than for a nest box, because a roost box accumulates droppings from repeated nights of use, and damp droppings mean parasites and mold. Scrape it out, scrub it with hot water and let it dry fully. Wear gloves and do the work outdoors, as with any bird-friendly yard maintenance job.

Then deal with the summer problem. A sealed, unventilated box in July sun gets dangerously hot, and if a bird nests in one, the heat can kill the brood. Either take the box down for the warm months, or open the ventilation and remove the perches so it functions as a nest box until fall. Do not leave a roost box sealed and available through the breeding season.