The rare capercaillie could be further threatened by tourism

The Swiss study found activities such as ski-ing, snow-shoeing and hill walking could harm the birds' fitness and their ability to breed.

The turkey-sized bird, which is a member of the grouse family, has been brought back from the brink of extinction in Scotland.

Numbers across Europe have also been dwindling as tourism figures rise.

Writing in the British Ecological Society's Journal of Applied Ecology, the study's authors, who carried out their research in the German Black Forest, said skiing activity could affect the bird's body condition and overall fitness.

Stress levels

The European ecologists collected the birds' droppings before and after the start of the ski season.

They analysed them for levels of the stress hormone corticosterone which they said was significantly higher in birds living in areas with moderate or high levels of such tourism.

They recommended that forests inhabited by the capercaillie should be kept free from tourism infrastructure and given special protection to address the problem.

Study author Dr Lukas Jenni, of the Swiss Ornithological Institute, said: "Ski tourism affects both habitat use and stress hormone levels in capercaillie, and this could adversely affect their body condition and overall fitness.

"Because of this, we recommend that managers keep forests inhabited by capercaillie free from tourism infrastructure and retain undisturbed forest patches within skiing areas."

Ski tourism affects both habitat use and stress hormone levels in capercaillie, and this could adversely affect their body condition and overall fitness Dr Lukas Jenni Swiss Ornithological Institute

In Scotland, where all ski-developments are above the tree-line of the Caledonian forests where capercaillie are found, hill walking activities could also be problematic.

A spokesman for the RSPB agreed that the birds were very sensitive to disturbance by humans.

"Our own research has shown that they tend to nest away from paths," he added.

"In Scotland skiers aren't a problem, but obviously walkers and hikers can be very disturbing.

"Our advice to people who are interested in capercaillie is to see them in the autumn and avoid them when they're nesting."

Just over 1,000 breeding pairs of capercaillie can currently be found in Scotland.