Many of the ducks are in eclipse plumage. Ducks moult all their flight feathers at once, and so for a month are unable to fly and are very vulnerable to predators. So the males moult and replace their bright body feathers by duller brown ones, much like the females. They will display this eclipse plumage for a few weeks or months depending on the species of duck, before another second moult occurs when the male will once again grow back his full colours. One way of distinguishing the species is to look for the speculum (coloured patch on wings) so the Teal in first images is showing the green speculum while the Mallard has just a glimpse of the blue speculum. No problem with identifying the Shovellers with their beak and the black-bottomed Gadwall.
