This shows a flock, also called a murmuration, of starlings at sunset. The hundreds or thousands of starlings are moving almost as if they are a single body, and mostly there is a pretty clear edge to the flock. It is pretty obvious where the flock ends and the rest of the sky begins. As the flock is a three-dimensional object, this should mean that the flock has a well defined surface, which separates the volume of sky occupied by the starling flock, from the surrounding sky where there are no starlings.
Murmuration is my new a favourite word, it means a flock of starlings. It is one of the old English collective nouns for a group of animals, like a murder of crows, a skulk of foxes or a gaggle of geese. And as the YouTube clip above shows murmurations are simply astonishing. The Guardian also has a gallery with some pretty amazing pictures. Thousands of starlings flying through the air as if they were a single organism. Flicking back and fore like cat’s tail, not like the thousands of bird spread across maybe 100 m that they are.