What's wrong with these leaves
Thank you for attaching the images. The fresh green tissue of your birch leaves has been eaten by small caterpillars called birch skeletonizers. Only the leaf veins remain.
Images and more are here http://www.extension.umn.edu/garden/diagnose/plant/deciduous/birch/leavesspots.html
Birch skeletonizer, Bucculatrix canadensisella, is a small moth but it’s their larvae (the youngsters) which do the damage. The caterpillars are less than a half inch long and feed, unseen, on the undersides of the leaves.
It’s too late to do anything this year other than rake up the fallen leaves. Because birch skeletonizer tends to appear on a 10-year cycle, it’s very likely you won’t need any pesticides next year.
If the tree is small, you could remove those little hungry fellows by hand; if much larger, you could hire a tree care spray service.
To obtain information appropriate to your region for this year as well as next, please contact the nearest county’s Extension Service office to ask when to act against birch skeletonizer,and what is an effective strategy.To locate contact information for the county office nearest you, please use this interactive map: http://npic.orst.edu/pest/countyext.htm