Huge flocks of more than a million common starlings may be observed just before sunset in spring in southwestern Jutland , Denmark over the seaward marshlands of Tønder and Esbjerg municipalities between Tønder and Ribe . They gather in March until northern Scandinavian birds leave for their breeding ranges by mid @-@ April . Their swarm behaviour create complex shapes silhouetted against the sky , a phenomenon known locally as sort sol ( " black sun " ) . Flocks of anything from five to fifty thousand common starlings form in areas of the UK just before sundown during mid @-@ winter . These flocks are commonly called murmurations .