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