Puffy trees

When we were in France, and driving out of Paris to Taize, we noticed these weird- looking things on some of the trees.

They look like masses of vines and branches and leaves. We thought they might be nests of some sort, but there were so many in each tree, I'm not convinced that would be the case.

Anyone out there have any guesses? Anyone? Anyone? Beuller?

Puffy trees

http://colleengleason.blogspot.com/2006/05/puffy-trees.html