Answer: It's really difficult to diagnose a tree problem without being able to physically inspect the tree. The questions I'd ask while looking at the tree are: 1) is the damage on the sunniest side of the tree? 2) are the stems and branches still alive? - use your fingernail to scrape off a little of the bark. If there is green tissue below the bark, the branch is still alive, which is a good sign. 3) are any other birch trees in your neighborhood similarly affected? 4) has it been hot and dry, and have you been watering deeply once each week during the growing season?
Heat and drought stress can cause early dropping of foliage and because it is the end of summer I suspect that's what's causing the problem. At this point rake up any fallen leaves and remove them from the garden. Be sure your tree is getting adequate water. Next spring it should leaf out normally. Hope so!
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