This phenomenon is called by the tender word murmuration.
Murmuration is a beautiful natural phenomenon, when thousands of birds gather in huge flocks, which dynamically form into amazing celestial figures, constantly shrinking and unclenching. The birds simply disperse and gather together like dancing couples of people or a crowd in a stadium. But, unlike the fans, these movements are not rehearsed everything happens spontaneously.
It seems incredible, but during the murmuration, a flock can unite even up to a million birds.