Mongrel Mallard
The title is Sheila's description of the odd looking duck masquerading as the father of a brood of ten ducklings on the mere. It's probably a juvenile from a previous brood that's sticking with its mother still, so it's not the father (which is what I first assumed) though the father is almost definitely an odd looking duck itself, that has passed on its genes to this one.
It's interesting to note that one of the ten ducklings is similarly coloured and stands out from the rest - it is all dark bar a white breast.
