Swifts are summer visitors to the UK. Last night I was out watching them over the river and in the fields beside it. I am unsure as to how many there were, but I think there were dozens. They move so quickly that counting much beyond three proved impossible.
It struck me that there are many balances involved in what swifts do. The balance between calories in from catching bugs and the energy needed to keep hunting the bugs. The physical balances in the air as they turn, swoop and dive, making what to me seem like very fast decisions about where to be. Several flew right past my head. For birds there are always balances around having enough weight to survive and not so much weight as to get in the way of the flying.
They flew close to each other, constantly in motion, their patterns of proximity changing all the time. It was hypnotic to watch.