In light of the follow-up discussion on this, and again with the caveat that I 
am not a shorebird expert, I offer one last post on these birds in case it's 
helpful to anyone planning to go to Blackwater soon.   Apologies to those not 
interested in this subject.

We saw two flocks that each had at least 20-30 birds--they were fast flyers 
and zigged around back and forth.  The first group never landed within view; 
the second did land for a little while at a moderate distance--not close enough 
for really good diagnostic scope views (at least not for me), but close enough 
to see a few things.

They were much smaller than nearby ring-billed gulls and looked "peep-sized", 
as opposed to the much larger and chunkier lone shorebird we had seen earlier 
and ID'ed as a Dunlin (which also had nearby ring-bills for comparison).  They 
looked longer-winged and a bit paler than I would expect for Leasts, but 
wouldn't swear to my ability to judge such subtleties for peeps.

Bottom-line--I wouldn't bet my life on it, but I really don't think they were 
Dunlin.  To me, Western seems like the best fit, with Least the second-best 
option.  I'm interested to see if anyone else goes and sees them!

Jason Waanders
NW DC
jwaandersATstarpower.net