The river runs beneath a stream of heavy traffic – two flows crossing: one threading quietly between the uprights of the flyover, the other thundering above with a sound like raging rapids. Hurrying under the eight lanes of A38 at the eastern edge of Plymouth, every strut and girder daubed with graffiti, I am keen to reach the greenery beyond. But something catches my eye, a movement close to the concrete elevations, a small bird flying low over the surface of the River Plym before settling in the stony shallows: a dipper (Cinclus cinclus). It takes me by surprise. This is a species I am used to seeing along Dartmoor’s tumbling torrents to the north. Here, opposite a retail park in the shadows of the Devon Expressway, searching for food close to a half-submerged traffic cone, it seems decidedly out of place. Dippers don’t just like water, they like it sparkling – clear rushing streams and fast-flowing watercourses, rich in oxygen and insect life. Yet, despite the urban setting, this section of river, snaking out into a muddy estuary far from its moorland source, evidently meets its requirements. With rotund shape, dark plumage and white bib, the dipper’s appearance hints at a photographic negative of a robin, though it is a stockier species, showing off its strength by repeatedly performing squats – the characteristic bouncing workout from which its name derives. Nudging into the current, it wades up to its belly, lowering its head beneath the surface to survey the sediment for invertebrate food, then slips underwater. As it edges upstream on the riverbed, ripples deflected by its back and wings give away its position, before it pops up and paddles across to the bank. A swimming songbird makes for a surreal sight, however much we know about its unique physical adaptations for aquatic foraging, such as a large preen gland for waterproofing dense plumage and flaps that plug the nostrils. Climbing on to a rock it resumes its energetic bobbing, bringing to mind an impatient waiter with white shirt front, dinner jacket and deferential bows. But something needs attending to and it dashes off downstream, calling as it goes, sharp, high-pitched, piercing the sound of cascading water and the traffic roaring overhead.