Not sure whether this is about the time they usually arrive - any other sightings?
The lengths some people go to: many years ago when I lived in the South East, housemartins built their nest under the eves of my house. On day upon returning from work I noticed that their mud nest had completely gone (remnants were on the ground below) and then saw that the adjoining neighbours (we weren't particularly pally) had made a crude nest out of orange terracotta clay under the eves of their house in an attempt to lure the birds to their house! The housemartins were very particular and chose to nest somewhere completely different as I never saw them again.
