Unknown bird

       Unknown bird

Observation - Unknown bird . Description: I really don't know what this bird is. It was sat on a tree in a large pond/lake. I thought it could be a Blackcap but now not so sure. If anyone identifies it, please could you tell me how you know i.e. what gave it away.

I really don't know what this bird is. It was sat on a tree in a large pond/lake. I thought it could be a Blackcap but now not so sure. If anyone identifies it, please could you tell me how you know i.e. what "gave it away".