This Swan was behaving very strangely

Twisting it's head around and flapping it's wings, but not taking off
It was only later when I looked at the images and realised what was going on
(see additional image)
Where I was standing there wasn't a single insect , but out there in the middle of the lake the poor bird was being besieged by a cloud of flies, which were invisible to me but the camera recorded them!