You dont mention what part of the country you are in, but in many areas this is
prime time for birch leafminer adults to be laying eggs. The adults are
actually sawflies, which belong to the same order of insects as wasps and can
easily be mistaken for them. They will lay thier eggs for the next few weeks
or so and then the eggs hatch, and feed in side the leaves. The "wrinkles"
are where the females deposited thier eggs inside the leaves. This is a
cosmetic problem and will not harm the tree.