This rare hybrid, also known as Sandberg's Birch, is very intermediate in most respects between its parent species.
This individual is about five metres tall, with several thick trunks.
The bark is darker than on Paper Birch, and is not peeling (The damage you can see at the left in the picture above appeared to be done by a squirrel or some other rodent).
Paper Birch has sharply pointed leaves with large teeth, while Bog Birch leaves are very rounded with small teeth. The leaves on this hybrid are pretty much right in the middle.
Bog Birch can also hybridise with Yellow Birch, but that can be eliminated by among other things the lack of a wintergreen scent to the twigs and that the apparent absence of Yellow Birch nearby.
Always fun to see something completely unexpected, and I certainly wouldn't expect that these two species with such different growth forms could produce viable hybrids!