Outdoors: River Devon, Clackmannan-shire

The River Devon, Clackmannan-shire, is a challenging prospect for the trout angler, with many parts of its banks dominated by overhanging alders and willows making casting and accurate presentation of the fly difficult. But the sheer mental concentration involved in making the short sideways casts required to avoid tangles is often relieved by the sight of a stunning bolt of electric-blue streaking along the course of the river like a mini cruise missile.

The kingfisher disappears in a flash as its whirring wings veer it round a bend in the river to find a favoured branch that overhangs a shallow languid pool popular with minnows and small trout. It is always a welcome sight which more than compensates for another fishless day on the river, but I fear it is also one that many Scottish anglers won't witness nearly so often for the next year or two.

The last River Devon kingfisher I saw was the previous autumn, and while kingfishers are notoriously elusive birds, as each angling trip draws another blank for catching a glimpse of this azure-winged beauty, the more convinced I become that the unusually cold winter has decimated the river's small breeding population.

It is well documented that kingfishers are vulnerable to prolonged cold snaps. In the Arctic winter of 1963, kingfisher numbers plummeted by up to 90 per cent, with other significant falls recorded periodically since then during similar cold spells. Kingfishers need to feed more than ever during cold weather, but frozen water stops the birds finding small fish. During a thaw, the resultant floods can last for days, with a kingfisher having little chance of sighting prey through a swirl of brown, silt-laden water.

Even if the water is low and clear, extremely cold weather throws small fish into a state of semi-torpor that makes them much harder to detect in sheltered parts of the riverbed.

Kingfishers are important indicators of the health of our rivers, and while there may currently be a temporary check in numbers, the overall picture is more positive. The birds need slow flowing water and are therefore largely absent from the fast flowing rivers of the Highlands, with the main concentrations focused in the central belt of Scotland and parts of the south-west. During much of the 20th century, some of these lowland rivers, including the River Devon, became contaminated with industrial pollution and agricultural run-off that reduced biodiversity. But improved water management in more recent times has resulted in an upturn in kingfisher numbers and they are now found in many of our river basin systems.

They can even be found within our cities, with kingfishers having been spotted in Glasgow, on the White Cart Water in Pollok Country Park, and in Edinburgh, where they are frequently seen on several stretches of the Water of Leith. This general upward trend in numbers in the last few decades has been aided by a prolonged series of mild winters.

If it turns out that Scottish kingfishers have suffered as a result of the harsh winter, then provided there are favourable conditions, there is every chance that the population will bounce back. Scottish birds are normally double-brooded and this fecundity may help replenish numbers. One of the biggest threats during the breeding season is nesting burrows in river banks becoming flooded out due to rising water levels caused by long periods of heavy rain, so a couple of dry summers would help speed recovery.

The Devon is a magical little river that has the ability to conjure up a range of amazing wildlife experiences, such as the dancing swarms of mating mayflies on a warm summer's evening, or the sight of the distinctive v-shaped bow wave of an otter as it swims across a deep pool. I have no doubt that kingfishers will soon return to the Devon to regain their rightful place as part of this rich tapestry, but until they do so, the river will just not seem quite the same.

• This article was first published in The Scotsman, Saturday June 5, 2010