 The call is mostly uttered in flight and is a harsh " tsak tsak <unk> " . The same sound , but softer , is made more <unk> when individuals gather in trees . When angry or alarmed they emit various warning sounds reminiscent of the <unk> thrush ( Turdus <unk> ) . The male has a rather <unk> song that he sings in the breeding season . It is a mixture of a few phrases like those of the common <unk> ( Turdus <unk> ) interspersed with whistles , guttural <unk> and call notes . This is sung on the wing and also from a tree and a subdued version of this song with more <unk> notes is sung by a group of birds at communal roosts . 
