Hitchin, Hertfordshire: Is it a mallard, is it a gadwall? In fact we have one of each, paired-up in an example of waterfowl hybridisation that isn’t unusual

We could be hiking through an upland ravine, miles from civilisation, were it not for the graffiti and half-submerged washing machine. Hart’s-tongue ferns hang down from the steep banks above us. The tang of fox rises from fallen hemlock stems, their dried umbels pointing towards the River Hiz. The water, smutty and lacking in vegetation, slides past an almost‑bridge – two brick abutments joined by an arch of sky.

Further on, the river widens round a bend and we’re greeted by a lemon flicker of undertail feathers. Dip, flick, dip, flick. A grey wagtail bobs on a rocky corner. Judging by the mizzle of midges round my head, the area must be a rich feeding ground for these insectivorous birds. I’m so enjoying the wagtail’s light-footed antics that I hardly register the mallards under the willows. It’s not until they drift towards me that I see a mazy grey tracery on the breast of one of the males, and that distinctive black rear end – a gadwall! On the river! He keeps pace with a female and, as gadwalls form pairs in autumn and winter, I assume he’s accompanying his less-conspicuous mate.