Pinkish-red growth on birch leaves are galls called erineum patches

Erineum galls are different looking than a typical gall on a leaf, which is more of a raised bump. The feeding of the mites causes the plant to produce abnormal growth, which in turn shelters the mites while feeding. Known as galls, the abnormal growth in this case are the pinkish-red patches you are seeing on the leaves of your birch tree.