The last of the Barn (European) Swallows that have graced our skies all summer set off northwards in early April. Departure signs appeared a week or two before. Usually during summer the swallows spend all day on the wing, returning to a communal roost, usually in a reed-bed, overnight. But as the days grow shorter, the swallows’ internal calendar tells them to get ready. As though they need to confer, they congregate in ever-increasing numbers along telegraph wires and fences. No Round-Table conference this, the birds line up, as though counting heads. Once a quorum is reached off they go.
The spacing on wires is interesting, it’s exactly 20 cm. There’s plenty of room for more swallows in the space, but any that try to perch there are shooed off. Like all animals (unless mating), swallows have a minimum comfortable social distance.