I've been wanting to take a photo of a Blackbird in this posture for a long time. Blackbirds do often cock their tails. When they alight they do that tail-cocking so as to keep their balance - in a similar way to Woodpigeons - they often do a jerky tail-cocking action when they do one of their various alarm calls 'chok, chok, chok' at the same time than they flap their wings. On Sunday, a windy day, this Blackbird perched on the side of a drinking trough, the wind made it lose its balance a bit and it stayed, tail cocked, for that second longer for me to press the shutter. Got you!