Brockwell Park: Aniridescent

More anatidanomalous in large.
Alternatives: Minkey washing, Preening cygnet, & Coots with maple leaves

The closer drake has abnormal colouration: his (normally) iridescent head and speculum plumage is brown! The iridescent colours are normally achieved not by pigmentation but by 'structural colouration', caused by the microstructure of the feather barbules interfering with the reflected light... His are just brown (the underlying pigment) rather than blue/green (which would be achieved by the structure of the feather). I'm not sure what this aniridescent aberration is called: it isn't any of the familiar ones (leucism, schizochroism, dilution...) because he seems to have normal pigmentation, with only those colours achieved by structural colouration being affected.
On a related note, if you ground up peacock feathers (destroying the microstructures) they'd just be brown too: most of their iridescent splendour is achieved by structural colouration rather than pigmentation.

Today's others are here (or right from Minkey stirring).