Troubled doubled Paper Birch
One side if our paper birch we planted as mere sticks (very fond of it) leaves out great in the spring, but within a few weeks, the leaves in the middle disappear. Very few turn yellow and fall off, and I can't see any bugs on it. It's been doing this for about 4 years. The other side looks great, but I have noticed what I assume are insect eggs on the underside of some leaves and chewed holes in leaves as well.
Wright County Minnesota birch tree leaf drop
The photos lack the detail needed to determine what's causing the symptoms you have described. However, you may be able to use this tool to diagnose the problem:
Otherwise, we suggest asking a certified arborist to assess the tree's health onsite and recommend a course of action.
Information at this website may also be of interest: