Bird song is normally associated with daytime birds, with only a small handful of notable exceptions.
Apart from owls, our other nocturnal songsters, corncrakes, nightjars and nightingales are all migratory birds with a short and well defined song period during the spring and summer months. As well as the true nocturnal species, reed and sedge warblers among others, sing extensively during the night.
All birds, whether diurnal or nocturnal, are governed by the daily rhythm of light and dark. Onset of song in the morning, the dawn chorus, is triggered by a combination of the birds internal clock and the very first rays of light.
The dawn chorus is normally started by the robin and the redstart, with sparrows and many finches being the last to join in. A very similar order, but in reverse, follows the sunset. It is thought that dawn chorus happens because birds wake up before there is enough light for them to feed and so they focus on singing instead.
Because even low light intensities can trigger song in some birds, and because they continue singing until the last rays of light have faded in the evening, it is easy to see how the singing period could easily be extended into the night. This is indeed what often happens with song thrushes and dunnocks, and doubtless many other species, but the unrivalled kings as daytime birds turned night-time songsters are robins.
Robins are insectivorous birds that are well adapted to foraging in dim light, and even continue to feed under artificial light well into the night. It is one of the earliest birds to start the dawn chorus and one of the last to stop singing at night.
With this tendency to be active at low light, robins can be easily triggered into full song by a streetlight or any kind of floodlighting. Since robins keep territories all year round, they also sing all round the year.
This has resulted in dozens of reports of nightingales singing in the middle of the winters night and other equally unlikely times and places, which have all turned out to be robins. In fact, the robin is the most common night-time songster in Britain's towns and gardens.
There are other triggers, besides light, that can bring about night-time song in robins and some other birds. If a bird is suddenly awakened by a sudden noise like thunder, fireworks, earthquake, wartime bombing etc, even a sudden shaking of its roosting tree, it may burst into song.
Robins can even be triggered to join in the singing of other nocturnal birds, notably the nightingale, to which it is distantly related.