I am so excited to tell you about this wonderful find. I was walking my pooch Rosie on our usual route down to the harbor and, dangling at eye level from a tree that I have passed a hundred times this winter, there was this structure. Thinking it was what it is, I ran home and checked my Lepidoptera books, and it is the cocoon of a member of the Giant Silk Moth Family, Saturniidaee (not to be confused with the oriental silk moth, Bombyx mori, from which silk fabric is spun).
Hanging from the tip of the American White Birch branch you could easily mistake it for a dry withered leaf, and that is exactly what the caterpillar has done, weaving the leaf around itself to pupate within. The cocoon is quite a good size, approximately two inches in length by one inch in width. The caterpillar pupates during the summer, overwinters in the cocoon stage, then emerges sometime in May or June. Giant Silk Moths live only for about a week. They mate soon after eclosing and then perish. Giant Silk Moths do not have mouth parts; all eating is done during the caterpillar stage.
Several members of the Giant Silk Moth family of caterpillars eat birch leaves. I am hoping (and it looks a great deal like) it is the cocoon of the simply stunning Luna Moth, however it could also be the beautiful Polyphemus Moth.