Mystery Nest!

Speaking of nests and recycling, check this one out. Yes, this is a nest. Made almost entirely of fishing line. I was in central Washington a few weeks back and was stymied by these balls hanging from trees alongside Wenas Lake. They looked so densely and solidly spherical that I initially mistook them for a human construction--some sort of visual conundrum/practical joke thrown up into the trees to confound passersby. But when my friend and I managed to pry one down to eye level, we saw that it was indeed a bird's nest: a rounded, snug cup comprised almost entirely of fishing line. It was lightly lined with plant matter and buttressed by a few sprigs.

The birds even added metallic dealiebobs which had a decorative effect--like earrings dangling in the breeze.

I'm not sure what species made these, so welcome any input.