A pioneering study of black-throated divers has revealed the special conditions this beautiful bird needs to thrive.
The UK population of the bird is restricted to remote lochs in north and west Scotland. Latest available figures show that it is scarce - the numbers stand at around 180 pairs and a new survey will take place next year – but it has previously been a mystery why some lochs are regularly occupied by divers, whilst others that are seemingly suitable are shunned.
The new research - based on a sample of 90 lochs in the Western Isles, Caithness, Sutherland, Strathspey and Badenoch and Wester Ross – demonstrates that fish populations are the key. Those lochs with a good supply of small brown trout and Arctic charr (the staple diet of the adult birds) and of minnows and sticklebacks (ideal for chicks) are most likely to support successfully nesting divers.
In lochs where these fish are scarce, then the birds are either absent or rely on insects for food and fledge fewer chicks. Lochs usually lack fish either because of unsuitable water conditions or because predatory pike have been introduced and have removed many of the smaller fish that the divers rely on for food.
“It is encouraging to see from this research that the signs are good for the black-throated diver in Scotland - providing its has access to the right kind of food supply,” said Dr Digger Jackson, the research scientist who carried out the study for RSPB Scotland. “Although many of the lochs that do not currently support breeding black-throated divers are clearly unsuitable thereare many others, possiblyup to100 in Scotland,where the food suppliesand shorelines are broadly suitable. These lochs have the potential to support new pairs of divers in the future, especially ifconservation measures could improve the food supplies andnest-sites forthe birds.”
Recent conservation work by RSPB Scotland, in partnership with other conservation bodies, has also shown the birds’ breeding success has greatly improved following the provision of man-made floating rafts which have proved ideal for nesting. These grass-covered polystyrene and wood structures are anchored to loch beds and help to overcome the problem of fluctuating water levels which can cause the failure of the birds’ nests on shore edges.
“The presence of black-throated divers is living proof of a healthy loch ecosystem,” said Dr Jeremy Wilson, Head of Research for RSPB Scotland. “Conserving this rare and special bird hinges on management of lochs. If they contain an abundance of small fish and good nesting sites and remain free of pike, then we can look forward to a secure future for the bird and an expansion in the population over the years to come.”