The Marsh Tit (Poecile palustris) on a mossy branch.
The Marsh Tit and Willow Tit are very similar, and the only way to tell them apart with a high degree of certainty is by their call. There are other subtle differences, and the Marsh Tit usually has a white mark on it's bill, and a glossy black cap.
If Marsh Tits find a good food supply, they may start to hoard seeds, burying and hiding them for a rainy day. Their hippocampus (the part of their brain which specialises in remembering things) is large, and bigger than other Tit's.