Mallard (Anas platyrhynchos)
The Mallard can be found anywhere there is water (salt or fresh), anywhere in Britain (both inland and coastal) and any time of year (winter and summer) and so, if you see a duck it is, statistically speaking, most likely to be a Mallard. That is why it is important to know a Mallard well so that you can know for sure when you have seen a different species worth a closer look.
I chose this photo I took of a Mallard because it shows very clearly the blue feathers in the wing. This is important because male and female Mallards are different in plumage but they both have the blue in the wing. In late summer the male moults and loses its gorgeous metallic green/blue head but usually the blue in the wing is still visible. To add to the confusion the Mallard inter breed with some forms of domestic duck and and all sorts of hybrids may be encountered but, even so, quite often the blue in the wing remains as clear indicator that you are looking at a form of Mallard.
There are other features too that identify a Mallard, one of which is their classic duck 'quack, quack, quack' call. The whitish flanks under the wing are quite clear, especially when in flight and so too is the white in the tail.
Get to know your Mallard well, then you will know when you are looking at a duck that is not!