The winter is an excellent time to estimate
Find a European Starling group on the ground
then count a portion of their flock.
When they get up
and come back down, then count a portion
Multiply the circle by the portion of those
Historically you will be reasonably correct.
I have been with birders who have come up with
the same number within 10 % several times
Lawrence Herbert, Joplin, MO. certhia
At att.net 01-22-14.