Bird-Proofing a Building at the Design Stage

Why do birds choose one building over another?

Shelter overhead, a level surface to stand on, and a cavity to nest in. Pest Prevention by Design, a guideline published by the San Francisco Department of the Environment with a technical advisory committee, puts it this way: semi-enclosed, horizontal roof surfaces, especially near outdoor eating areas, are invitations for pigeons. The Internet Center for Wildlife Damage Management makes the matching recommendation from the control side, which is to modify structures and architectural designs to make them less attractive to pigeons.

That gives a designer three questions to ask of every exterior detail. Can a bird stand on it? Is it sheltered from above? Is there a hole behind it? A detail that answers yes to two of the three will be used.

What should ledges and sills look like?

Sloped and smooth. Pest Prevention by Design says to slope smooth-surfaced window ledges and projections at 45 degrees to minimize perching and roosting, and to design decorative screens, moldings, siding, awnings, sills, signs, sprinkler pipes and column capitals so they offer no place to perch, roost or nest, especially near entrances. Its summary instruction is to use smooth materials and avoid horizontal surfaces.

The 45 degree figure is consistent across sources. ICWDM says roosting on ledges can be discouraged by changing the angle to 45 degrees or more, and the U.S. Department of Energy's Building America Solution Center repeats it for homes, recommending sloped rather than horizontal ledges, column headings and fixtures. On a brick storefront or a metal commercial building that can be as simple as a sloped cap flashing where a flat one was drawn.

Where do beam ends, trusses and canopies go wrong?

Exposed structure under a roof is the single most reliable bird attractor we see, and it is a favorite of western Montana architecture: timber-frame entries, log purlins, open web trusses over a fuel island or an arena, heavy beam tails under a deep eave.

Pest Prevention by Design addresses it directly. It advises avoiding semi-enclosed alcoves and courtyards, which it says provide ideal roosting and nesting opportunities for pigeons and other birds, and where they must be included, building in bird barriers and minimizing horizontal surfaces. It recommends that openings, exposed rafters on overhanging dock roofs and other likely perches in semi-enclosed areas be screened with rust-proof wire or plastic mesh of three quarters of an inch, or half an inch to exclude rodents as well. It notes plastic netting is less durable and needs replacing more often.

If the look depends on exposed framing, decide at design stage how it will be protected. ICWDM describes netting fixed to the underside of the rafters, with access panels so lights and equipment above can be serviced. Fixing points and a tidy perimeter line are easy to provide during construction and awkward to add later. Smooth soffits under a canopy, or bevelled tops on beam tails, remove the perch altogether. Mud-nesting swallows have their own requirements, covered in swallows nesting on a Montana building.

How should signs and light fixtures be chosen?

By the shape of the top. Pest Prevention by Design says to choose exterior light fixtures with sloping surfaces rather than horizontal ones to deter roosting and nesting, and lists spikes, netting and similar devices as the fallback where a flat-topped fixture is already in place. The same logic applies to sign cabinets and channel letters: a flat top is a perch, and the gap between a cabinet and the wall is a nesting cavity for house sparrows unless it is closed.

People often ask whether the warmth of a fixture draws birds to it. We did not find that addressed in the design guidance we reviewed, so we will not put a number or a study behind it. The guidance is about geometry, and geometry is what you can specify. See bird spikes and ledge deterrents for what gets fitted when the fixture cannot be changed.

How should vents terminate?

With a durable cover on every one. The Building America Solution Center recommends corrosion-proof wire mesh with openings of one quarter inch or smaller over vent openings, sturdy soffit vent covers mechanically attached to framing so wind and animals cannot dislodge them, and a stainless steel chimney cap with screening. It cautions that mesh finer than half an inch significantly reduces air flow and can clog or freeze over, and that air handler intakes should not be screened finer than half an inch unless the manufacturer says so.

Clothes dryers are the exception that matters. The same guide says to cover dryer exhaust openings with gravity louvers and not to install screens over dryer terminals. A louvered hood that closes fully is the specification. Our guides to starlings nesting in vents and birds in dryer and bathroom vents show what happens when that detail is missed.

Do roof and gutter choices matter?

They do. The Building America guide recommends simple roof forms, metal step and valley flashing, and leaf guards over gutters, to discourage the buildup of nesting material and the rot that opens a way in for animals. Both it and Pest Prevention by Design call for bird stops in the open ends of rounded roof tiles.

Rooftop equipment deserves the same thought. A solar array or a mechanical unit with a sheltered gap beneath it is a new semi-enclosed space, and it should be planned with its edge protection, not fitted with it after the first nesting season.

What about the refuse area and the grounds?

Food and water bring birds to a site before the building keeps them there. For refuse areas, Pest Prevention by Design recommends heavy-duty containers with tight-fitting lids, a concrete floor sloped to a drain, and a hose bib nearby so the enclosure can be washed down. ICWDM adds eliminating pools of standing water that pigeons use. Grade paving so it drains, and keep seating areas clear of overhead perches.

Is it better to build it in or retrofit?

Build it in. Pest Prevention by Design is candid that retrofit devices such as looped wires, spikes, springs and nets are not foolproof and require maintenance, and it flags that bird barriers can be unsightly and expensive to install after the fact. A sloped ledge needs no maintenance and nobody notices it.

Retrofitting is still what most owners face, and it works when it is scoped properly. That is our bird netting and exclusion service, and what to expect from a bird exclusion project walks through the process. The guideline's last relevant point applies to both: provide access to enclosed spaces so they can be inspected, because a void nobody can reach is one nobody checks.

What to ask your designer or builder

Ask for an elevation review with birds in mind before the drawings are final: every horizontal surface wider than a hand, every exposed member under cover, every vent termination, every sign and fixture, and the refuse enclosure. Ask who is responsible for vent covers and for closing gaps at eaves and sign backs, since those items fall between trades.

We are not architects or roofers, and we do not design buildings. What we can do is review plans or a finished shell from the point of view of the people who get called afterwards, as part of a bird mitigation assessment. The rest of the series is in the Montana Bird Control Guide.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a bird assessment and we will identify the species, the access points, and what will actually stop them.

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