Martins gathering

Each evening house martins (and the occasional swallow) gather in large numbers around the roofs of the bungalows, on the wires and on the road.  We counted around forty birds tonight.  

What attracts them we are not sure, but the roof tiles will still be warm after a sunny day and there may be insects in the moss.  Or perhaps its just a social gathering, making plans to escape our summer for better climes.