I had a most amazing sight when coming out of Waitrose in Sudbury at about 4.30pm last Tuesday, just as it was getting dusk.
The air was suddenly full of birds. They circled round, wheeling, scattering and then reforming the flock, before descending on to the supermarket roof.
They were all pied wagtails. There must have been between 150 and 200 birds, but difficult to count, as they were continually taking flight again, only to reland a few moments later.
I think it probable that they roost there regularly, so keep your eyes to the sky when you next visit Waitrose.