[JWPalmer]

Those are Bald Faced Hornets, they eat yellow jackets. We have them here, and I watched with interest as one had a fight with a yellow jacket, then proceeded to eat it. Was really neat to watch. They can have HUGE nests.

Yep, and JohnM got it right regarding building materials. I had a lilac bush darn near killed from the hornets girdling the bark.The hornets eat bees too so I do not have much use for them. Eradicate.