The sky over my garden was buzzing with House Martin (and a few Swift) tonight, all performing the same strange flight patterns. And the reason? Well, a quick look at my records from this time last year provides the clue.
Last year it was Starlings not House Martins but they were doing exactly the same thing - catching the large number of flying ants which have evidently just hatched. Due to the slow airspeed of these insects, the birds catching them behave in an unusual manner, slowing right down to pluck them from the sky, almost stalling as they hover for a few micro seconds to feed. And it is not just Starlings and martins that behave in this manner - I have seen House Sparrows do likewise on other occasions (also when the Mayflys first appear over the nearby River Leam, a month or so earlier).
The similarity of habit between these very different birds species is amazing, as is the coincidence of date. Last year I noted the Starling behaviour over my garden on July 27th, and today's display was of course on July 28th.