Mud nest ID
Aren't they incredible?
That is the paper nest of the Bald-faced hornet. They are considered beneficial and people often don't notice them, their activity or nest until after the leaves fall. It is not reused from year to year.
If you wanted to take it down to look at it, we'd wait through a few more days of cold weather.
Here is our page about them: http://extension.umd.edu/hgic/bald-faced-hornets
Thanks so much! You learn new things every day!! Now I have to do some research.