Sort sol is a nature phenomenon in the marshlands in southwestern Jutland, Denmark, Very large numbers of migrational starlings gather there in spring and autumn when they move between their winter grounds in southern Europe and their summer breeding grounds in Scandinavia and other countries near the Baltic Sea.
Sort sol takes place in the hours just after sunset. The birds gather in large flocks and form huge formations in the sky just before they decide for a location to roost for the night. Sort sol in the marsh near Tønder can occasionally comprise a formation with up to one million birds. Usually flocks break up when the number of individuals exceed about half a million birds due to excessive internal disturbances in the flock.
