House Martins and Swallows both arrived early last spring and started nesting ahead of schedule. They are both double-brooded and leave soon after the second brood juveniles are capable of long-distance flight. So this would have occurred earlier than normal.
Secondly, the prevailing northerly winds were unusually persistent and strong in August, providing favourable conditions for southward migration. My local Pallid Swifts also left early and may also have been taking advantage of the ‘free ride’.
Alan Vittery, By email