Increasing day length (photoperiod) sends signals to a bird's brain and a whole cascade of hormonal changes are initiated in response. One result is the onset of reproductive behavior, such as increased singing associated with establishing territories and pair bonds. In other words, increasing day length gives birds raging hormones. I would hypothesize that night time singing 1) is an artifact of heightened hormones and/or 2) birds are responding to the additional light supplied by street lights.

Mike Donahue
Seattle