Have you ever noticed that male blackbirds practice their spring song through the winter, but very quietly? We first noticed a few years back when we had a nest of blackbirds in our barn. One male stayed on over his first winter. As the season progressed, we noticed a tiny song, usually coming from a hiding place and if we poked around we would find him tweeting away. Closer to spring he became ever louder and his song more complex as his confidence increased. Finally he was happy with his skill and went outside to sing from the tree tops.
This autumn/winter I have already noticed two different blackbirds doing it, well hidden in the hedges. At first I thought they were singing at a normal level from far away, albeit out of season, then I realised both were just a couple of yards away, and were concentrating so hard on their music lesson that they didn't notice me. It's very sweet