Perhaps they're...playing chicken *tired ba-da-dum-tssss*
Seeing who can rush through the door before it closes
(I'm joking though)
Is there one who's consistently the one left out? Perhaps she's low in pecking order, or staying out longer to eat?
Well, some low pecking order birds may go in first to get a safe spot maybe...
I've just seen some birds go in long before others to get their preferred spot, and other birds stay out a long time. One of mine often stays out longer to eat, when everybody else is settled.