I found this video lurking on my computer, taken a few years ago at Otmoor nature reserve, near Oxford, and thought it was worth posting.
It's a swarm of an estimated 50,000 starlings, who roost in the reed beds there over the winter. Some of what seems to be background noise in the video is actually the noise of the starlings flying over. There's also a little rain of droppings as they pass - we had umbrellas with us!
Note: it's worth switching the video to the best quality - after starting it playing, click where it says "360p" at the bottom right of the player, and change it to "480p".