Right now, any birdsong you hear (songs, as opposed to alarm calls or contact calls) will almost definitely be a robin. They are one of the few city birds - few birds anywhere, in fact - to sing through winter.
The blackbirds and wrens, the dunnocks and thrushes, all are quiet now. Their chicks have flown and their territories are abandoned until spring, their focus simply on staying alive. Many, for instance the great tits, blue tits and long-tailed tits, will flock together through the long, cold months.
Not so the robin. Both sexes hold territories and defend them all year round. That thin, silvery song you hear pouring down from the street light at dusk, or carrying reedily to you over the sound of the early morning traffic: that's a robin. I'm here, he or she is saying; I'm alive. This is my manor. Deal with it.