The last two evenings from about 6:55 to 7:25pm I've watched a 
continuous stream of black birds flying low overhead southward along the 
Lemon Fair River lowlands toward the river's extensive flooded lands 
west of West Street in Cornwall.  I assume, but do not know, that there 
is a temporary night rookery there, perhaps in a tract of floodplain 
forest on the west side of the lowlands in Bridport.

The streams each evening consisted of flock after flock ranging from 
10-15 birds to 60-80 birds per flock.  Three to five flocks passed by 
per minute.  My crude best estimates made from adding up the flocks 
yielded a count of at least 3000 birds, and possibly several hundred more.

The birds were well silhouetted against the twilight sky, and as the 
birds were flying just above the treetops they were easy to scan with 
binocs.  The vast majority were Common Grackles.  Well interspersed were 
birds that appeared to be Red-winged Blackbirds or perhaps even Rusty 
Blackbirds.  I would not have been able to distinguish Cowbirds.  I did 
see a very few Starlings.  And trailing one group of grackles there was 
one cluster of four birds with the body shape of Woodcocks and the 
flight paths of inebriated bumblebees.

The birds were almost entirely silent, with only exceedingly scarce, 
isolated "kek"s.  Both nights the streams ended quite abruptly, after 
which I never saw even a single straggler.

Southern end of Snake Mountain, Cornwall