A chickadee comes in to the feeder, quickly grabs a seed, and flies away. It may return immediately, but it's more likely to wait its turn.
When a whole flock of chickadees moves into the yard, it looks as if they form a living conveyer belt. One chickadee after another flies to the feeder and leaves with a seed.
The birds take turns rather than squabbling over the seeds. The Cornell Lab has some great tips about feeding chickadees - including this Chestnut-backed Chickadee - and birds in general.