This ID is even less obvious than the chaffinch. It's heavily modified, but I'm confident all the same.
This ID has been confirmed! The Paul McCartney Project gathered lots of information about the design work for The Boys of Dungeon Lane, including some images from instagram.
The garganey (or summer teal) appears in front of John's face. Below is the bird from the red collage, the green collage, and the garganey (flipped) from Coloured Figures of the Birds of the British Islands.
Below I have pasted one bird on top of the other (first the red collage, then the green collage) and shown the combination at different opacities. Further down I have the 0% and 100% illustrations (in both colours) with circles indicating points of comparison. I hope this helps explain why I think this is the right bird, but I'd love to hear your thoughts.
