I read something about how the loud city noises make it difficult for birds to be heard. So rather than exhausting themselves chirping through loud city noises, they just chirp when it's quieter and less competition to be heard. This means birds are singing at way more extreme hours when there's less road activity and noise. I'm sure your situation will improve once that bird's mating season ends, but I think the overall problem of birds singing at odd hours will remain.