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I have a double-hung window which has an AC unit, meaning it is always open. Occasionally, like just now, small birds get trapped in between the panels and they can't get out because it is narrow and they have like 15" to climb with glass on either side. Is there any way to put something on the window that will prevent small critters from getting trapped when there is an opening they can get through?

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up vote 4 down vote accepted

The standard approach to sealing the top of the lower sash raised for the AC is a foam strip placed between the outward facing uper edge of the inner sash and the outer glass pane.

window insert

This gap is on the inside of the house.

The birds you mention must be getting in through the lower gap between the lower rail of the upper sash and the glass pane on the inside sash. There is no reason you could not use the same type of foam insulator on the outside (assuming you can safely reach the window). While this is not a hard barrier that will prevent a forceful assault by a determined attack bird, it probably will be enough to discourage the accidental intruder.

The foam will likely deteriorate much more quickly outdoors, but should last a season.

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I use a pair of hand towels or a small bath towel, rolled up tightly. Leave the loose edge within reach, or you'll be asking how to retrieve the towel ;-)

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I take it one step further and put a rubber snake on top of the a/c unit. – mikes Jun 16 '13 at 20:33

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