Sort sol

Sort sol is a murmuration, a flocking behavior that occurs in the marshlands in southwestern Jutland, Denmark, in particular the marsh near Tønder and Ribe.

The birds gather in large flocks and form huge formations in the sky just before they decide on a location to roost for the night.

The movements of the formations have been likened to a kind of dance or ballet and the birds are so numerous that they seem to obliterate the sunset, hence the term "sort sol" (Danish for "black sun").

Sort sol in the marsh near Tønder can occasionally comprise a formation of up to one million birds.

In rare cases the sticky deposits may render the predator unable to stay airborne.