I lunched at Loch Raven Reservoir today.  In the pool below the dam,
there were a group of approx. 30 ducks that I've not been able to
identify.
 
What I think were the females, looked like common or red-breasted
mergansers with a beautiful reddish-brown "crown".  However, what I
thought were the males seemed more like ring-necks except they were
larger than mallards and the bill appeared black with a double white
ring at the tip.
 
Can anyone help me identify?
 
Also, at the parking area above the second bridge, there appeared to be
buffleheads or male hooded mergansers in amongst a few mallards and the
usual Canada geese and gulls.
 
(forgive me, I'm not yet an experienced birder)