There are swallows and house martins around Biddestone, and I walked up to West Kennet Long Barrow from Avebury this afternoon, and there were a bunch of swallows flying around there, because they nest in a cavity in one of the burial chambers as you go in. If you stand at the back of the chamber opposite, and keep quiet, they forget you’re there and start flying in and out.
Not a good photo, but tricky circumstances, the nest is tucked up in the top right corner.