WATCH: Huge flock of birds heads south in 'The Black Sun' phenomenon

Take a look at this incredible video out of Denmark.

Millions of migrating starlings taking off at the same time and heading south for winter.

The phenomenon is known as the "Black Sun" because in some cases the birds block out entire sections of the sky.

Thousands of people head to a national park on the North Sea to catch a glimpse of it every year.