Interesting!

Audobon Society put a warning against using yarn scraps for nesting material. The yarn cannot break and causing injury. Birds can get tangled in the fiber.

Shiela, these nests are being used at a rescue center where the birds are monitored. Not for out in the wild.

For the birds, though, whenever I comb my cats, I pull the fur off the brush and put it outside. One time I found a bird nest and it was lined with my cat's fur. :)