My experience with birches is limited since I only have two. But branches may die if they are wired too hard, pruned too hard or for no reason at all. Nowadays I wire very carefully just before leafing out. The only time I prune branches is from late spring until the middle of the summer. This means that the tree will be very ugly all other seasons, but it seems that fewer branches die. The weeping style is also very good for birches, as long as you have some long branches you can always wire them down to hide all the other ugly branches
/magnus