I was there also on Sunday morning (3/23, 9:30 to 11:30). Saw
about the same birds except for the Red-breasted Mergansers,
instead found a few other duck species there: American Widgeon (2),
Blue-winged Teal (4), Red-headed Duck (1), Wood Duck (7).  There
were also a few Rusty Blackbirds around.

One interesting thing I noticed was a large bloom of Fairy Shrimps in the
ponds at HH.
I dont know the species, it is a type of freshwater crustacean, related to
brine shrimp - the Sea-monkeys of the good old days, :).  As I had never
noticed this before  I dont know if this was exceptional or not.  I wonder
if the prolonged dry period that those ponds suffered during late summer and
early fall has anything do with the proliferation of the Fairy Shrimp.
I suppose this might be one reason those ducks are hanging around in large
numbers for so long.

Dalcio Dacol
Poolesville, MD