It's starlings in murmaration.
Millenium Faken.
https://www.youtube.com/watch?v=Vy7yuj-UrNI
I'm only posting this because I love the music.
And to say it definitely can't be that entirely natural thing that we know exists because of my spurious reasoning about that natural phenomenon.
Far more likely to be aliens than my lack of knowledge of starlings.
I vote fake. Too many videos with the same angle, same sequence but from different locations. If that were birds we should be seeing different shapes, different sequences, morphing into additional shapes at some point.
It's fake.
As SusanB-M1 pointed out upthread, it's a publicity stunt for Eden Project in Cornwall. Her link didn't work so I Googled it.
I love that.Birds or no birds, CGI murmuration is apparently pretty straightforward as digital special effects go.
Here is an example, created by DDB NZ, used by NZ Sky TV a year or two back to promote the introduction of their "Sky On Demand" service, and set to the awesome piece "Outro" by M83
I suggest full screen to get the best effect
https://www.youtube.com/watch?v=MrXIenYX3iE