Zeedman
Garden Addicted
Glad you pointed this out @flowerbug ; I too have found yarn in our bird house nests. You can't underestimate the power of a determined bird to untie that perfectly colored string.we've had birds swipe yarn used to tie things. years later i may find bits of it used in nests that have fallen out of trees.
But the biggest ball of string I've come across was inside a squirrel nest. They had apparently been chewing up a burlap sack.