In the fall and spring, thousands of people come to the marshlands of the Wadden Sea, between Denmark and Germany, to watch huge flocks of migrating starlings take flight at sunset. The birds synchronise their movements in a flocking behaviour called a murmuration. They swoop, shift, and turn as one, creating incredible shapes in the sky. The Danes call this phenomenon 'sort sol,' or black sun. And the description's apt. The starling flocks can be so thick they appear to darken the evening skies. Experts believe murmuration is a defense against hawks and other raptors, or it could also be a way for the birds to stay warm as temperatures dip. Whatever the reason for their mesmerising, undulating movements, one thing is undeniable – the starlings put on a stunning show.