A SMALL dark object swirls on the surface of the pool by the edge of the River Devon, writes Keith Broomfield.
I am momentarily confused as it looks like the size of a duckling or moorhen chick but it is too early in the season for it to be a baby waterfowl. Perhaps it is a water vole? I slowly swing the binoculars up to my eyes, quickly revealing that it is in fact a dipper, a songbird about the size of a small thrush that actually swims!
Although at home in the river, the dipper is certainly not an accomplished surface swimmer with its lack of webbed feet making its movements somewhat erratic. Then it dives underwater before reappearing a few seconds later to alight upon a large midstream boulder, its vivid white breast conspicuous as it bobs up and down.
Similar in shape to a large wren, the dipper is a real specialist of our fast flowing rivers and burns that can forage underwater on the streambed in search of food items lurking under pebbles, making good use of a rich niche not exploited by other songbirds.
Dippers have linear territories along a specific stretch of a river and if you walk along a water course a dipper will keep flying away from you until it reaches the territorial boundary when it will then turn back and fly past you. Such is its affinity with the river that it rarely ventures more than a few metres away from its course.
A typical riverbed is home to a multitude of invertebrates for the dipper to feed upon. A sweep with a fine-meshed net of the shallow margins of a burn will soon reveal this hidden bounty. There are tiny shrimps and nymphs are plentiful too - the first or larval stage of the remarkable lifecycle of winged insects such as mayflies, caddis flies, stoneflies and gnats. The nymph can spend two years living in the river feeding on tiny plants and animals before emerging as an adult fly.
Usually a dipper walks or plops into the water from the river edge or a rock before disappearing where it uses its wings to exert downward thrust to counteract its natural buoyancy, enabling it to forage for such small creatures along the bottom.
I have noticed that the dippers on the River Devon tend to swim on the surface like a duck only in winter and when it is very cold. I’m not sure why this should be the case, but it could possibly be due to aquatic invertebrates moving into deeper water during such conditions, meaning the dippers themselves have to venture further out into the river before diving down to reach them.
Dippers are one of our earliest breeders, with eggs usually laid by early April. It is thought that they nest early so that the hatching of chicks and development of fledglings coincides with a peak in abundance of aquatic invertebrates in the river in May and June.
The abundance of aquatic invertebrates such as mayflies and stoneflies is dependent upon water cleanliness and the presence of dippers is a sure sign that all is well with the river.