Male nightingales arrive on the breeding grounds ahead of the females and begin singing immediately. Night song functions as long-range advertisement to females arriving overnight on migration, when the acoustic environment is quiet and a song carries much further than it does over daytime noise. Daytime song, by contrast, is used mainly in territorial exchanges with neighbouring males.
The split shows in the birds themselves. Studies of colour-ringed populations find that nocturnal song is sung overwhelmingly by unmated males and drops sharply or ceases once a male is paired, resuming if his mate is lost. Paired males are not wholly silent after dark, but they sing far less than unpaired neighbours, so a wood that rings all night in early May can fall quiet after dark in June while daytime song continues. The song itself is unusually elaborate, with individual males commanding song-type repertoires numbering in the low hundreds.
A common misreading is that any bird singing at night is a nightingale. Robins routinely sing after dark, especially near street lighting, and in Britain they are the more likely candidate outside the nightingale's shrinking south-eastern range.