There is an outstanding amount of information at that link on the science of birch sap and the techniques for its harvest and for protecting the trees, and the subtleties of rendering it down to a syrupy consistency.
I haven't decided yet whether to undertake that aspect of the adventure. Everything I've read suggests the process is time-consuming and needs to be undertaken with some degree of care to avoid scorching the concentrate. I have about a half-liter of fluid now, because the trees are still dripping into this third day (memo to self: in the future don't prune when the sap is running).
Read more about birch sap at TYWKIWDBI. Link