What's The Birdie

The fisherman has got an idyllic peaceful spot by this lake in a remote part of Epping Forest. I started chatting to him with a view to getting a close up but sensed that I was an intrusion. I left him to his efforts and on reaching the other side of the lake there was this group of ducklings with parent duck (extra). But can any of you Blip Bird Spotters tell me what it is? It's got a little flash of yellow on its wing. Is it a Teal? But looking in my bird book teals don't have that yellow flash detail.

Have we seen a rarity?