Hi, We did see a lot of birch trees last summer losing foliage as a means of coping with the dry weather. As better weather returns, they normally will put out more new foliage. Part of what we saw last summer was the impact of several seasons of poor growing conditions as well. I suspect, assuming no insect or disease symptoms, that your tree has suffered greatly. It is not uncommon to find the finer, smaller twigs and branches dead after a winter. If the trunk of the tree and main branches are still viable, you will see sprouting along the trunk and larger branches.
This can happen well into late May. I would not write this tree off until you give it some more time. If you scrap the bark on some of the limbs, you should see a good dark green color that is wet looking. This tells you that branch is alive. If the color is a light or pale green and looks dry, that branch is failing. If the canopy went into the winter months without enough water in the stems and buds, winter dieback would be the result.
Trees must grow every year or they will not survive. Care for all birch should include watering monthly throughout the dripline, as birch come from areas of high water tables, near creeks, streams, lakes, etc.
You can contact a local arborist to do an examination, to let you know the true condition of your tree.
I hope this information is helpful.