I thought Oak Treehoppers, but, since they were on Black Birch, I didn't know if that might also be a foodplant, despite the name, or if there was another similar species that fed on birch. Thanks for the quick ID!

The photo was taken October 11. When the trees go dormant, what do these creatures eat when sap isn't flowing? Will they be adults by then and do they overwinter in that form, or will they have already reproduced and the eggs are what overwinters? Or...?

My source for saying they feed on birch was Kaufman's field guide, however I just noticed that BugGuide's description page [http://bugguide.net/node/view/538] says "Habitat

Forests and forest edges, parks, and anywhere Oak trees are found. Occasionally found on other trees, but these individuals were probably just resting on those non-Oak trees."

It also says that they're active in the winter in the South, but probably not in the North.

So, hopefully someone more in the know will be able to answer your questions.