Sort sol

Sort sol is a remarkable natural event that looks like a dark sun filling the evening sky. It happens in the marshlands of southwestern Jutland in Denmark, especially near the towns of Tønder and Ribe. During the spring and autumn, huge numbers of migratory starlings gather in these marshes as they travel between their winter homes in southern Europe and their summer breeding areas in Scandinavia and around the Baltic Sea.

This amazing sight occurs just after sunset. The starlings form enormous, shifting patterns in the sky, moving together like a dance or ballet. Because they are so many, they can block out the sunset, making the sky look like a black sun—hence the name “sort sol,” which means “black sun” in the Danish language. Sometimes, as many as one million birds can be part of this beautiful display, although usually the flock breaks up if it grows larger than about half a million birds.

When a predator bird approaches, the starlings work together to defend themselves. They may throw droppings and vomit to coat the predator’s feathers, sometimes making it hard for the predator to fly. This clever behavior helps keep the flock safe as they find a place to rest for the night.