I going to wade in here, mainly because I want to learn about the key features between Eastern Marsh and Pied and would hate to waste such a great set of pictures.
Rather than actually giving any answers, I can only supply more questions and hope for more light on this poorly described area.
1.The solid black primary tips and the relatively unbarred pale area on the underside of the primaries, I think are features of Pied rather than Eastern Marsh but I may be talking out of my other orifice.
2.I have no idea how to age or sex this bird.I'm very interested in why the rear half of its body is so dark? Are these juvenile feathers???
All comments are greatly appreciated.
eric