I returned a week ago from my Maine Section hike and I didn't see much about this place on WB so I thought I'd add it.
The Birches is Baxter State Park's campsite for Long Distance hikers (If you completed the 100mw you qualify) and beginning SOBO's. It consists of a wooden tent platform (~20x20), 2 6-person lean-to/shelters, a large picnic table, a nice firering bench, and a pit toilet (with toilet paper). You are only allowed to spend 1 night and the cost is $10 per person. Collect water at the Katahdin Stream by the Ranger Station, that's where you'll register. The ranger station also sells firewood. BSP is a bit weird in that there are no trash cans (that I saw anyway), water faucets, electric outlets (that I saw) or vending machines. Nor did I see a campstore. So if you are entering NOBO, buy your favorite beverages at Abol Bridge.