I heard a Least Bittern calling at the Oxbow Lake Preserve in Anne Arundel County this morning.  If you follow the trail to the right from the entrance, you can see the tall grassy area it was in from the last regular observation point.  You can get closer if you bushwhack a bit further around the edge of the lake.

I took a loop around Lake Artemesia in PG county afterwards, where a Pied-billed Grebe is still hanging around.  

John Hubbell
Washington DC