Do you have a sparrow spooker on the box? Cowbirds fit in Troyer boxes (slotted boxes) but it is unusual. They tend to pick on ground/low nesting birds like warblers, Song Sparrows etc. I’ve had an influx of Cowbirds this year – first time….I found an egg in a Chipping Sparrow nest. Unfortunately the egg “broke” as I was looking at it closely ;)
