Cassin's and house finches mob the sunflower-seed feeder.
I took the photo two days ago, one of our Project Feeder Watch days. You can see two male Cassin's airborne on the left. Counting these guys is like trying to count the participants in a riot.
Even worse are the pygmy nuthatches, who move in a continual loop from the feeder to a tree branch and back again, giving a sort of "cloud of electrons" effect.
When M. and I are counting, we try to monitor two sunflower-seed feeders like the one pictured, two tube-type Niger thistle feeders, one hanging suet-cake feeder, and one small window feeder -- and check who is hanging out in the adjacent trees.
Sometimes I cheat a little with the finches. Female house finches can be told from female Cassin's, but for me, at least, that requires a little study with binoculars--hard to do during a major session of sunflower-seed looting. So I count the males and multiply by two.
It's just grunt-level science.