Nightingale Population Going Down - The Nightingale, in Urdu Bulbul, is a small passerine bird which is named so because they frequently sing at night as well as during the day. A study by the British Trust for Ornithology (BTO) revealed that numbers reduced by more than nine in ten between 1967 and 2007. This is the largest recorded fall of any bird still breeding in Britain except the tree sparrow (93 per cent).