Nightingales were small brown birds of thrush kind, famous for their clear singing, and the fact that they would sing during the night as well as the day (hence their name).
Names and Etymology
Nightingale in Sindarin is dúlinn or merilin. The Sindarin poetic word for 'nightingale' was tinúviel, the name Beren gave to Lúthien when he first saw her. They were also called the "Birds of Melian", circling above her head while she sat in Menegroth.