another option if you dont want to spend money on a converter, or losing your quality by having to crop a whole bunch is to zoom in
. Yes i said ZOOM IN, obviously your lens cant do it anymore fully extended at 200mm but all that means is that you have to do the zooming for your lens. Use your feet and get physically closer to the birds (without scaring them away that is). Now if you were already standing at the edge of the water, maybe getting a canoe or some other kind of boat and sitting out in the middle of the water there