What Causes Birds to Start Singing?

It was a murky morning today but I heard several species singing: Skylark, hedge accentor, greenfinch, chaffinch, song thrush, robin. Plus a drumming woodpecker. The only difference that might have sparked this irruption of song has been a slight rise in the temperature. Obviously the days are also getting longer, but that would not exp;lain the sudden burst of birdsong today compared with previous days.