Window strikes: the 3 ft / 30 ft rule and how to stop the collisions

The short answer

• Put feeders closer than 3 ft to the glass or further than 30 ft away. The middle distance is the dangerous one because it gives a startled bird just enough room to reach full speed.
• Every treatment goes on the outside of the glass. Decals stuck inside are hidden behind the reflection you are trying to break up.
• Marks must cover the whole pane with gaps no larger than about 2 in. A single hawk silhouette in the middle of a window does nothing.
• A stunned bird goes in a dark, ventilated box in a quiet place, with no food and no water, and is released within an hour or two or handed to a licensed rehabilitator.
• Collisions with glass are one of the leading human-related causes of bird deaths in North America, and almost all of it happens at ordinary houses rather than towers.

Glass is invisible to a bird, and a reflection of your maple tree is worse than invisible: it looks like somewhere to go. Collisions with windows are one of the leading human-related causes of bird deaths in North America, and the great majority happen at houses, not skyscrapers. One or two panes on an ordinary home can account for nearly all of it.

The fix is cheap and almost entirely mechanical. It comes down to two decisions: how far your feeder sits from the glass, and what the outside face of that glass looks like.

Distance changes the speed, not just the frequency#

The standard advice is to keep feeders under 3 ft from a window or more than 30 ft away. That rule is usually explained as "fewer birds hit the glass", which is only half of it. The real mechanism is impact speed.

A chickadee leaving a perch is not at cruising speed. It accelerates over the first few wingbeats, and it needs a stretch of clear air to get there. Put the glass at arm's length from the feeder and a spooked bird meets it while it is still pushing off: it bumps, drops, and usually flies away. Put the glass 15 ft away and the same bird arrives at something close to full flight speed.

That distinction matters more than it sounds, because the energy of an impact scales with the square of the speed. A bird moving twice as fast does not hit twice as hard, it hits four times as hard. The difference between a survivable knock and a broken neck is often just a few extra feet of runway.

Distance interacts with cover. A feeder 20 ft from a window with a shrub 6 ft away is worse than it sounds, because the birds bolt for the shrub and the window is on the way. Work through where to put a bird feeder before you start taping things to panes, since moving the feeder is free and permanent.

Why treatments must be on the outside#

This is the single most common mistake, and it makes an otherwise good effort useless.

A reflection forms on the outer surface of the pane. When the outside is brighter than the inside, which is nearly all of daylight, that outer surface behaves like a mirror. Anything you stick to the inside sits behind the mirror. You can see it perfectly from indoors, which is why people believe it is working; from the bird's side it is hidden under a bright image of sky and trees.

So: tape, dots, cord, paint, screen, film. All of it on the outside face. If a product cannot survive weather, it is the wrong product for this job. The only treatments that legitimately go inside are interior blinds and shades used to kill the see-through tunnel effect, and they only help when they are actually closed.

Spacing: the 2 in rule#

Birds do not read a window as an obstacle, they read gaps as flyable. So the goal is not to decorate the glass, it is to leave no gap wide enough to attempt.

The working rule is that no untreated gap should be larger than about 2 in in any direction. Older guidance allowed 2 in horizontally by 4 in vertically, on the reasoning that birds are less willing to squeeze through a narrow vertical slot. The 2 in grid in both directions is the version to use if you want one number to remember, and it is the one to use on any pane that has already killed a bird.

The corollary is that a single decal does nothing. A hawk silhouette in the middle of a 4 ft pane leaves several square feet of untreated glass on every side, and the bird simply goes around it.

The "lasts" column is a working estimate for an exposed pane; a sheltered north wall does better.

Two of those deserve a note. Screening is the most effective retrofit most people can make, because it removes the reflection and cushions any bird that still arrives. It is also the least attractive, so it tends to end up on the one bad window rather than the whole house. Cord curtains work because they move: a swinging vertical line is read as a physical obstruction, and they are nearly invisible from inside.

One-way film and other partial answers#

Reflective privacy film reduces daytime reflection on the room side and increases it on the outside, which is the opposite of what you want. Films sold specifically for collision prevention carry a visible printed pattern, and it is the pattern doing the work, not the film. Judge any film by one test: stand outside at bird height in daylight and see whether the marks are clearly visible with gaps under 2 in. If they are not, the film is a heat product, not a bird product.

Two other partial measures are worth doing anyway. Close interior blinds on panes that line up with a window on the opposite wall, which removes the see-through tunnel. And turn off unneeded outdoor and upper-floor lights during spring and autumn migration, which is a night-time problem rather than a feeder problem but costs nothing to fix.

Moving the feeder to the glass instead#

The under-3-ft half of the rule is a real option, not a consolation prize. A feeder suction mounted on the pane, or hung from a bracket within a foot of it, converts the window from a hazard into a viewing hide. The birds that use one are the small perching species plus a few larger birds if there is a tray to stand on.

Top top-tier favorite Yes eats readily Some takes occasionally Rare rarely No not a feeder food

Fill it with sunflower hearts rather than seed in the shell. There is no ground under a window feeder to absorb hulls, and a sill full of wet shells becomes a hygiene problem fast. Full detail on the design in window feeders.

Finding your bad window#

Do this once, on a bright morning, and you will know where to spend your effort.

1. Walk the outside of the house at about 5 ft, roughly the height a bird leaves a feeder at.
2. At each pane, note whether you can see a clear image of sky, trees or shrubs. Sharp reflections of vegetation are the highest-risk signature.
3. Note whether you can see straight through the house to a window or glass door on the far side. That tunnel is the second-highest risk.
4. Look for evidence on the glass: a smudge shaped like a wing, a dusty print from feather powder, a small smear at chest height.
5. Rank the panes. Treat the top one or two completely rather than treating every window partially.

Partial coverage of many windows is the classic failure. One fully treated pane prevents more collisions than a scattering of dots across the whole house.

A stunned bird: the first two hours#

Most birds that hit glass and survive the impact are concussed, not broken. They need dark and quiet, and nothing else. Handling, food, water and light all make it worse.

1. Get it out of danger first. A grounded bird is helpless in front of a cat. See cats and feeders for why that risk is immediate.
2. Pick it up gently with a cloth or a bare hand cupped over the wings, and put it in a paper bag or a cardboard box lined with paper towel. Do not use a towel with loops, which snags claws. Punch a few air holes and close the box.
3. Put the box somewhere dark, quiet and room temperature. No radiator, no sunny car, no room with a television. Do not check it every five minutes.
4. No food, no water. A concussed bird cannot swallow reliably, and water offered to a bird in that state can be inhaled. This is the instruction people most often ignore.
5. Test after 20 to 30 minutes. Take the box outside to a sheltered spot near cover, open it, and step back. A recovered bird leaves at once.
6. If it is still down after an hour or two, or shows a drooping wing, blood at the beak, an eye that does not track or a head held to one side, it needs a licensed wildlife rehabilitator. Search your state or provincial wildlife agency's rehabilitator list, or call a local nature center or veterinary clinic for the referral.

Handle any bird as little as possible and wash your hands afterwards, which is standard practice around wild birds for the reasons set out in diseases at feeders.

The permanent version of the fix#

Every treatment above is a patch on a piece of glass. The structural version is to change what the glass reflects. A window that faces a shrub layer, a small tree or a trellis 2 to 3 ft out reflects far less open sky, and it gives startled birds somewhere real to go. That is one more argument for the planting work in native plants and cover, and it is the only collision fix that also feeds the birds. If you are laying out a new station from scratch, decide the window question before you sink the pole, as covered in starting a feeding station.

Frequently asked questions#

How far should a bird feeder be from a window?

Either less than 3 ft or more than 30 ft. Inside 3 ft a bird that panics off the feeder cannot build up speed before it meets the glass, so a bump replaces a fatal impact. Beyond 30 ft the feeder is no longer part of the same visual scene as the window, and birds treat the two separately. The gap in between is where the damage happens: enough runway to reach flight speed, and the glass still in the escape path.

Do window decals stop birds hitting the glass?

Only if there are enough of them and they are on the outside. A reflection forms on the outer surface of the pane, so anything behind the glass is invisible from a bird's side in daylight. Spacing matters as much as placement: birds will try to fly through any gap bigger than about 2 in, so one or two shapes in the center of a large pane leave a wide open target on every side.

What do I do with a bird that hit my window and is sitting there stunned?

Move it out of reach of cats into a paper bag or a cardboard box lined with paper towel, close it, and put it somewhere dark, quiet and room temperature. Give it no food and no water. Check it after 20 to 30 minutes by opening the box outdoors in a sheltered spot; a recovered bird will leave immediately. If it still cannot fly after an hour or two, or has a drooping wing, a bleeding beak or an eye that does not track, contact a licensed wildlife rehabilitator. Do not keep it.

Does one-way reflective film work?

Partly, and only in one direction of light. Films that let you see out while showing a mirror outside work when the outside is brighter than the inside, which is most of a sunny day. They fail at dusk and at night with lights on indoors, and they do nothing about the fact that a mirror is exactly what birds fly into. Patterned external film with a visible 2 in grid is a more reliable choice than any film sold purely for privacy or heat.

Why does the same window keep killing birds?

Almost every yard has one bad pane, and it is usually one of two types: a large window that mirrors sky and trees, or two windows on opposite walls that create a visible tunnel through the house. Go outside at bird height at about 8am and look at each window. If you can see a clear image of your own trees, or straight through to the far side of the house, the birds see the same thing. Treat that pane first and you fix most of the problem for the cost of one roll of tape.

Sources and further reading

1. All About Birds news and guidance Cornell Lab of Ornithology Why birds collide with glass and how to break up reflections
2. Bird Feeding and Care National Audubon Society Feeder placement relative to windows and cover
3. Migratory Bird Program US Fish and Wildlife Service Legal protection of native birds and why care requires a permitted rehabilitator
4. Birds Canada Birds Canada Canadian collision monitoring and homeowner guidance