Hi Kara,

This is a bald-faced hornet nest. While the queens have moved out to overwinter elsewhere (not in the nest), there may be workers who are still in the nest. It's been there all summer and you're just seeing it now because the leaves have fallen. 

Once we get consistently freezing weather (low 20s at night), you could safely remove the nest without getting injured (late December/January). Since it's high in the tree, you would need to hire an arborist or someone with the proper equipment to remove it.

Best option: just leave it - the hornets don't use the same nest two years in a row. Eventually it will tatter and shred. Bald-faced hornets are not as aggressive as yellowjackets and usually won't string people or dogs. Bald-faced hornets can be considered a beneficial insect in that they reduce populations of unwanted insects (including other yellowjackets) and will help pollinate flowers when they are searching for nectar.