One morning last spring, I noticed that my freshly washed windows were covered with odd-looking smears. Not being particularly compulsive...

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One morning last spring, I noticed that my freshly washed windows were covered with odd-looking smears. Not being particularly compulsive about clean windows, I left the puzzling stuff there.

That same afternoon, strange rhythmic sounds began emanating from downstairs. Click. Tap. Tap tap. Thump! The search for the sound ended up at the very site of my polluted windows. There was a robin purposely colliding with my dining-room window. The same sound then began upstairs. Another robin? No, this time it was the tree swallows (Tachycineta bicolor) in the nest box on our house.

Although it seemed as if the whole avian world had declared war on my house, such attacks are a common ritual of spring. The birds were vigorously defending their nests against “intruders” — in this case, their own reflections in the glass.

This behavior occurs frequently with American robins (Turdus migratorius) and less frequently with spotted towhees (Pipilo maculatus), chipping sparrows (Spizella passerina), song sparrows (Melospiza melodia) and swallows. Sometimes it persists a few days, sometimes weeks. Sometimes it will stop and start again days or weeks later, possibly because the pair is nesting a second time.

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Both male and female robins defend territories, although males do so more often. What must be crazy-making is that the reflection does not behave even close to the real thing. Normally, robins can easily chase away a male trespasser. Reflections aren’t so cooperative. Every time the actual robin gets closer to the window, the “other” robin also comes closer. When the real robin assumes a dominant posture, the reflected robin takes the same stance.

Perplexed, the real robin leaves, perhaps going to his favorite perch to sing. Hearing no response, he returns — to find the supposed intruder still there, so the “fight” resumes. The bird may exhaust itself, but the battering usually does not result in major injury or death.

Remedy: Eliminate the reflection. Closing the drapes doesn’t work; neither does hanging fake owls, hawk silhouettes or static-cling decoration on windows.

A bright light inside the window sometimes reduces reflection. Floating helium balloons outside the window at about robin level sometimes works. Taping strong thick paper or opaque plastic painter’s dropcloth over the outside of the window, or soaping the entire outside of window usually works, as do bird screens (http://wdfw.wa.gov/wlm/living/robins.htm).

When all else fails, wait it out. But don’t wash your windows until after the nesting season.

Patricia Thompson is a wildlife biologist with the Washington Department of Fish and Wildlife. She can be reached at thomppat@dfw.wa.gov.