We have a very large bird feeder,18 inches in dia. and 10 inches deep, clear plastic with a bowl shaped cover over it hanging in a Birch Tree , we fill it with sunflower seed.
I was letting it get empty so I could wash it out.
I hadn't looked at it for a few days till my wife noticed a couple of days ago that she couldn't see through it. Upon closer examination we found it to be completely filled with Bald Faced Hornets, [the most aggressive and largest of the lot ] building a very large nest, almost filling the entire feeder.
The other night well after dark, I got the largest garbage bag we had, sprayed the inside with " WASP and Hornet Killer " and bagged the whole outfit and placed it in the sun in my utility trailer all taped up tightly so they can't escape.
It is amazing how quickly these hornets can invade and build their nests .
I will let it sit for another day or so then take it out and clean up the whole mess.
Googling Bald Faced Hornets, there are pictures of some nests that are absolutely huge.