As seen on Minnetonka Boulevard: This is not a nest. It is an odd growth of branches called "Witches' Broom". The University of Minnesota Horticulture Department assured my wife that it is not the result of jet contrails, or nuclear radiation. The cause can be tree fungus, insects or de-icing salts.
I tend to prefer the traditional explanation: Witches flying overhead have caused this broomlike oddity to grow in the trees.