Dency and all,
Although not frequent, Cowbirds have been observed backing up to Bluebird nest boxes to successfully drop an egg in! Bluebird nest boxes most commonly have a 1.5 inch round hole (although other appropriate entrance hole shapes exist) so a Cowbird could not fit in unless something has enlarged the entrance, even just a bit. A House Wren box would typically have a one inch round entrance hole and a box for Carolina Wrens or other small birds would have a 1.25 inch entrance hole or larger. Carolina Chickadees can utilize 1.025 inch openings.
I would think it would take much greater skill for a Cowbird to plop an egg into a 1 inch entrance hole, but do let us know if it happens!
Steve Garr
Missouri Bluebird Society
Jefferson City, MO
Cole county
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From: Dency Kahn <[log in to unmask]>
To: [log in to unmask]
Sent: Sunday, May 7, 2017 8:23 AM
Subject: Bird behavior

I have a wren box presently tenanted by house wrens.  Yesterday there was a female cowbird fluttering at the entrance, I assumed to attempt to lay in the nest, an exercise in futility.  I chased her away but she came back.  Can't bigger birds tell they can't get in?

Dency Kahn
Olivette

Sent from my iPad
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