The mallard is so common in the UK, such a successful survivor, that I often overlook it when taking photographs. But it really is a rather beautiful bird.
Particularly striking is the iridescence (surfaces that appear to gradually change colour as the angle of view or the angle of illumination changes) in the head and neck of the male. This was particularly apparent at WWT Slimbridge recently, with the colours on full show for the breeding season, and the sun still quite low on an April afternoon.
The male appears sometimes green, sometime blue, and colours in between.
It’s not just that the back of the head is blue and the side green, see for example this otherwise undistinguished shot of resting males. The apparent colour really does depend on the angle of view and of the sun.
Because of the easy availability of food, mallards are very tame at Slimbridge, so you can get close. These shots are of the female, also attractive but more by patterning than by colour.