Certainly the largest mixed flock I have every seen was just off Wildlife Drive near Egypt Road. But remember that is a much visited spot which of course increase the probability that I'd see anything there. It was late fall or early winter several years back.  Also once I saw male Red-winged Blackbirds (only) in huge numbers and great density perching on tree limbs at the south end of High Island, on the Potomac where a feeder canal feeds into the main Georgetown section of the C&O canal.  In such great density that the tree limbs were being weighed down. Accompanied by a cacophony of noise. It was startling. It must have been in the sixties and most likely in the Spring since I remember thinking migration as an explanation -I wasn't a birder then.  In the years since I have never seen anything remotely like that again either along the river or anywhere else.

Phil Brody