Here they are illuminated by powerful lighting. As the wagtails jostle for position there is a loud and sustained twittering chorus, but this performance fades once any latecomers have put in appearances. The birds do not huddle together even on frosty nights. The communal roosting habit of pied wagtails has been widely studied. At sewage works the wagtails will even roost carousel-fashion, on the rotating arms at treatment centres.
Many sites are traditional and have been regularly occupied for years.
Roosts may contain tens, hundreds sometimes thousands of pied wagtails. Some roosts are occupied all-year with the largest numbers between August and March when residents and migrants roost together. Others are used only in autumn, or spring and autumn. Again traditional, over 30 years is the longest known period of occupation. There are several advantages in using such sites.