Why Do Birds Attack Windows and Mirrors? Two Problems, One Piece of Glass

There are two completely different things a bird can do to a window, and almost every page online treats them as one.

One is a bird flying into the glass at speed, once, and often dying. The other is a bird coming back to the same pane every morning for a week to fight something it can see in it. They have different causes, different seasons, different levels of seriousness, and almost opposite fixes. Sorting out which one you have is the whole job.

The short answer

If the bird keeps coming back

It is attacking its own reflection, because it has decided there is a rival in its territory. Spring and early summer. It usually stops on its own within days to a few weeks

If the bird hit the glass once, hard

That is a collision. The bird saw sky or plants reflected and flew at what looked like open space. This is the serious one

Which is more common

Collisions, by an enormous margin. Cornell cites a 2014 study putting the figure at up to about a billion birds a year in the United States

The fix that matters for both

Whatever you put on the glass has to go on the outside. The reflection lives on the outer surface, so anything taped inside changes nothing

Two problems that look alike

The attack

Repeated, deliberate, often from a perch a short distance away. The bird flies at the glass, backs off, and returns. Sometimes for hours a day

The collision

One impact, at flight speed, usually with no approach and no repeat. Often first thing in the morning or around dusk

What the attacker sees

Another bird of its own species, in its territory, refusing to leave

What the collider sees

Sky, or the trees behind you, or the plants on your windowsill. It does not see glass at all, because glass is not a thing evolution prepared any bird for

Season

Attacks concentrate in spring and early summer. Collisions peak with migration, and Cornell notes they cluster at dawn and dusk

Which birds attack, and why yours

Cornell names the usual suspects. American Robins, Northern Cardinals, bluebirds, California Towhees, Chipping Sparrows and Song Sparrows. Both males and females do it. Audubon adds Rufous Hummingbirds, which will take on a car mirror.

What they have in common

They defend a territory aggressively and they nest low enough to be around your house all day

Why the same window every year

Because the same bird, or a bird holding the same territory, is facing the same reflective surface at the same point in its breeding cycle

Why a car mirror

It is a small, sharp, eye level reflection parked inside a territory. From the bird side it is the same problem in a smaller frame

The species pages

If it is a robin, the behavior fits the pattern in the robin article. Cardinals and bluebirds show up in the cardinal guide and the bluebird guide

Does it hurt the bird

Our two sources put this slightly differently, and the difference is worth keeping rather than smoothing over.

Cornell writes that reflection attacks are seldom a threat to the survival of the bird, while also saying the bird can exhaust or even hurt itself and is being pulled away from things that matter. Audubon calls it typically not harmful, and draws the same line we are drawing here, between this and the high speed collisions that are often fatal.

The honest reading is that both are true. A bird spending three hours a day fighting a pane of glass is not usually going to die of it. It is also a bird that is not feeding, not guarding a real nest, and not resting. That is a cost worth ending, which is why the fixes below are worth ten minutes of your time rather than none.

How to stop an attacker

1. Work on the outside of the glass. Only the outside. Both sources say this and it is the single most common mistake. A sticker on the inside is sitting behind the mirror.
2. Kill the reflection rather than decorating it. Cornell lists netting, fabric or newspaper over the outside, or soap streaked across the pane. Audubon adds sheets, cardboard, an exterior screen or a mylar balloon anchored outside.
3. Expect it to look bad for a week. Cornell puts the behavior at a few days to at most a few weeks. You are covering a window for that long, not forever.
4. Fold the car mirrors in, or bag them. Audubon suggests paper rather than plastic on a parked mirror, because plastic can mark the clear coat.
5. Do not bother with a rubber snake. Cornell says outright that birds get used to seeing them. Anything stationary and harmless stops working once the bird learns it is harmless.
6. Do not chase the bird. It will come back, and you will have added yourself to the list of things it is worried about, for no benefit.

The collision problem, which is the serious one

If your bird hit the glass once and hard, none of the above applies. This is a different problem with a much bigger number attached.

The scale

Cornell cites a 2014 study estimating up to about a billion birds a year killed by window strikes in the United States

Where it happens

Not mainly skyscrapers. Cornell is explicit that far more birds are killed at low rise buildings and houses, simply because there are so many more of them

Why the glass is invisible

The bird sees the sky or the vegetation reflected in it, or sees straight through to plants on the far side, and flies at what reads as open air

The spacing that matters

Markings need to be spread across the whole outer surface, about 2 inches apart. A single hawk silhouette in the middle of a large pane does close to nothing

Other options Cornell lists

Paracord hung outside at about 4 inch spacing, an external screen covering the full pane, or a one way transparent film that reads as opaque from outside while you can still see out

A mirror is not an intelligence test

It is tempting to read a bird fighting its reflection as proof the bird is not very bright. That is not what it shows, and the research on this is more interesting than the joke.

Recognizing yourself in a mirror is a specific and rare ability. The one bird that has produced a positive result on it is the European magpie, in a 2008 study where two of five birds tested reacted to a colored mark on their own throat after seeing it in a mirror. That is covered in the magpie article, including the controls that made the result count.

So a cardinal attacking a window is doing exactly what almost every animal on earth does with a mirror. It is also worth noticing what this behavior does not tell you. A bird that cannot identify itself in glass may still be very good at identifying you, which is a separate question with its own experiments, gathered in the article on which birds have actually been tested.

What this has to do with a bird bath

More than you would expect, in one narrow way. A still water surface is a mirror.

It is a weak one, because it is horizontal and a bird approaching a bird bath is looking down at it rather than head on, and because any movement in the water destroys the image. Reflection attacks on water are not a thing anyone reports, and we are not going to invent them.

The real overlap

Glass sided and highly polished basins near a window can add a second reflective surface to a spot where a bird is already agitated

The easy fix

Moving water. A dripper, a small pump or a wiggler breaks the surface, and a broken surface reflects nothing

The placement point

A basin directly in front of a large window puts the birds you want at the exact distance where a startled takeoff ends at the glass. Near cover is better, for this reason and several others, covered in the placement guide

The window part

If you keep a basin near glass, do the outside of that window properly. It is the one place where feeding birds and window strikes genuinely intersect

Frequently asked questions

How long will the bird keep doing this

Cornell says a few days, or at most weeks. It is tied to the breeding cycle, so it ends when the territorial pressure ends, whether or not you do anything.

Will covering the window make it worse

No. The bird is responding to an image. Remove the image and there is nothing to respond to. What makes it worse is a partial job, because a bird will keep working the part of the pane that still reflects.

Why does putting decals inside not work

Because the reflection forms on the outer face of the glass. Anything inside is behind the mirror from the bird point of view, and the bird never sees it clearly enough to matter.

Is a bird tapping at my window a sign of something

It is a sign that it is spring and your window is shiny. There is a large body of folklore attached to this, and it is worth knowing that the behavior has a plain and well documented explanation underneath whatever meaning you take from it.

The bird hit the window and is on the ground. What now

That is a collision, not an attack. Many stunned birds recover if left undisturbed in a quiet, covered spot, but a bird that does not recover shortly needs a licensed wildlife rehabilitator rather than a well meaning amateur. We are not qualified to talk you through the rest of it, and we are not going to try.

What we could not check

What we read in full

The Cornell Lab answer on a bird deliberately attacking a window or car mirror, the Cornell Lab article on window collisions and prevention, the Audubon piece on the same behavior, and the 2008 magpie mirror paper

The tension we left in

Cornell says the behavior is seldom a threat to survival and also that the bird can hurt itself. We reported both rather than picking the tidier one

The number we are passing on rather than verifying

The billion a year collision figure comes from a 2014 study we did not open. We are citing it as Cornell reports it

What we are not claiming

That reflective bird baths cause attacks. Nobody reports it and we are not going to be the first, on no evidence

References

1. Reference: A bird keeps flying into my window or car mirror, on purpose. What should I do? | The Cornell Lab of Ornithology, All About Birds
2. Reference: Why Birds Hit Windows and How You Can Help Prevent It | The Cornell Lab of Ornithology, on collisions, scale and prevention
3. Reference: Why Birds Attack Windows and Mirrors and How You Can Stop Them | Audubon magazine
4. Reference: Mirror-Induced Behavior in the Magpie (Pica pica): Evidence of Self-Recognition | Prior, Schwarz and Guentuerkuen, PLOS Biology, 2008