Bob - I knew it was an usual sighting, but didn't realize it was that
rare!  And in retrospect I realized that I didn't provide any
information about how we identified them as Whimbrel.  When I first saw
the birds (with my naked eye) my initial impression was a small gull -
they were flying in a line formation and I could see that they were
stocky birds with pointed wings and a steady, fairly shallow wingbeat. 
But when I got them in my bins, I could immediately tell they were dark
and not white like gulls, and immediately determined they were large
waders.  I suspected Whimbrel during the first, brief sighting, though
couldn't say so unequivocally.  It was only after I got them in my scope
(Swarovski, AT80HD w/zoom lens), my suspicions were confirmed.  The
bills were medium length and markedly, obviously decurved.  To give a
sense of overall size, they seemed to be about the size of a Bonaparte's
Gull.  As for color, they were a fairly uniform medium brown throughout
(though slightly paler in the belly/vent, but not the tail).  These
features, combined with the call, a tootling whistle, immediately
confirmed Whimbrel.  Based on these features, the only remotely similar
species to be considered are other species of curlew and godwit. 
However, the length of the bill was too short for Long-billed Curlew and
the curvature ruled out any species of godwit.  I guess Bristle-thighed
Curlew is the only real contender in terms of size/shape/color, though
even that species, as improbable as it is to be seen in the east at all,
let alone in such numbers, can be ruled out by both the call and the
fact that the rump and tail were not lighter than the rest of the birds.


Aside, here's a clip from a post by Kurt Gaskill from May 17, 2009
regarding a field trip to Dyke Marsh:
"I think the weather scared many off as only 5 showed up for the first
joint ANS-FODM Field Trip to Dyke Marsh.  Yet, the participants who
arrived were quite pleased as a huge flock of Whimbrel was on the
Hunting Creek mudflats, riverside. Since the light shower mostly lifted
by 8 am, the flock was ready to continue migration and so would aloft to
the air and fly about, then land again. Then up again. Finally, they
took off and I counted 130 Whimbrel with 15 Short-billed Dowitchers
mixed in.  They definitely flew over Jones Point lighthouse and veered
over the river flying over the Wilson Bridge (putting a few wings firmly
into DC airspace).  After the regular Haul Rd Field Trip, a few of us
went up to the Stone Bridge and viewed 17 more Whimbrel along with 2
Black-bellied Plover, a Ruddy Turnstone, 3 Dunlin and 5 more
Short-billed Dowitchers.  An amazing sighting and unprecedented in No. 
VA (or DC) as far as I know."

I happened to be at Hains Pt at that time, but never saw a single
Whimbrel (let alone 147!).  But I didn't know to be on the lookout for
them, as I didn't find out about them until later that day.  And as I
recall, viewing conditions were not as good as today, so it's possible
they were lost in the clouds and never viewable from Hains. 

Between these four occurrences, it appears that the period between May
17-27 is the time to be looking for Whimbrel in DC.

Good birding,
Paul Pisano
Arlington, VA