Geese don't count, Faldage. That isn't to say that they can't count, but geese don't roost in trees. At least they don't down south when they are on vacation.

And Inselpeter, Yes, I think that the polarized vision of birds influences many aspects of their behavior. One such influnce could be the effects of polarized light as the sun slowly drops beneath the horizon - the green flash of cruse ships. Is this flash the signal for the flock to rush in unison to the safety of the overnight roosting trees? Or are they simply responding to a subtle movement of a leader with their keen eyesight as Maverick has suggested.