Withybush Wagtails

Yesterday evening, just before dusk, we noted the silhouettes of numerous pied wagtails perching on the hospital roofs. By chance, a sparrowhawk passed over while we were there, spooking them all into the air in two large and quite impressive flocks. 

We estimated that there were at least 600 wagtails in the air at one point before they all started to re-settle. 

We could not watch them for very long unfortunately, but it is likely that other wagtails were still arriving to join the roost. However, when we returned about half an hour later when it was getting darker, we could not detect any wagtails roosting at a usual location. Perhaps (as we have noted previously) the entire flock had moved off to roost elsewhere?