You see herds of them queuing up to fly south for the winter.
But, do you ever see them coming back again?
I think the problem is that they are staying in Australia. This accounts for the RSPB saying that the number of wild birds in the UK is decreasing. It's because they keep flying away and not coming back.
And, I think there may be some sort of global warming link. You know, all that flapping in one direction only, changing the rotation of the earth and all that. I'm still working on this bit though.