Growth of yellow leaved birch

  Hi, Can anyone help identify the strange growth that has just appeared on my golden birch tree?.

The tree is very healthy atm so it doesn't look like it is damaging the tree ( I am sorry I could not get closer to take a better picture). Some did suggest it could be the start of a hornets nest as a few have been buzzing around the tree, but they are further up near the top of the tree.

I would be grateful of any advice.

Thanks