Oh I know how you feel, all bent over. My young maples did the same thing. The time to straighten up the damage is in early spring using splints and duct tape but if the trees are young and you are able to reach the branches you may still be able to straighten them out with sprints. For large trees I would just accept the new look or wait until spring and have an arborist come prune off the damaged branches for a fresh start. I have seen birch trees roped and pulled more upright with the ends of the rope tied to stakes in the ground. You can retrain tree branches over a few years of staking and tying but the real challenge is reaching and securing large branches on tall trees. It is not worth risking injury by climbing a ladder into your birch as these trees are known for their weak wood - and falling ladders are the result. You may be better off enjoying the bend. Keep growing, Marianne Binetti