I don’t know much about birds, but that looks like an exotic/pet bird to me — maybe a parakeet or budgie?
I looked at the Craigslist pets section to see if there were any corresponding listings for lost birds. I didn’t see any, but you might want to check periodically at http://washingtondc.craigslist.org/pet/ .
There was a listing for a lost cockatiel that mentioned the following “things to do” if anyone saw the lost bird — if you see the 6th/Emerson bird again, these might be applicable (even if “responds to whistles” isn’t):
“[R]esponds to whistles. Please try to contain, entice with seeds, cover with towel & put in covered box.”