In reply to stoo:
Ah, that explains it. We saw exactly the same thing yesterday - millions of seagulls flying around together. We ARE near the sea, but I've never seen that many together at one time. No rubbish tip, no fields being ploughed (which was my first thought), must have been the flying ants. Must admit, I didn't thing seagulls ate insect, I thought they were more fish / carrion / general rubbish feeders. But the ant theory makes perfect sense