It is fairly regular at this time of year to see large gatherings of Pied Wagtails at dusk time.
Where I work, an area that has large areas of industrial roofs and large areas of tarmac, interspersed with grassy areas, it has been a popular haunt for them as long as I can remember.
It is great to watch them doing their little floating upwards hover type manoeuvres, and seeing them running at high speed on the ground, their little legs pounding faster than an Olympic sprinter.
Supermarket car parks are another place to watch out for them too - both of the main ones in Stratford have regular flocks.
You will also be able to readily see the difference between mature adults and this years juveniles from their plumage too.
Check out the details in the Collins Black Book - link below if you don't have it. An essential for UK and European Birds.