Culprit Identified!

        Last spring I mounted three nesting boxes miles from the previous ones. Two of the boxes were observable when I accessed this property. Never did I see any activity of birds around the the two boxes. Today when cleaning the boxes the reason was clear. A  wren had stuffed those two boxes full of twigs. (See the photos below.) The third box had been used for nesting. "The male house wren will fill the neighboring nest boxes with twigs to prevent other birds from using the boxes – these are called dummy nests & you can remove them."  Internet

Takk for alt, 

Al