A secretive animal that shows itself only at night is on the increase in West Dunbartonshire.

Pine martens are said to be thriving in woodland north of Dumbarton and around the south shore of Loch Lomond.

However, their nocturnal habits and shy nature means that you have to be lucky to see one - and only then it will be a fleeting glimpse.

A group which studies these shy creatures, which are relatives of stoats and weasels, says the indications are that numbers are on the rise.

Lizzie Croose from Vincent Wildlife Trust (VWT) said: "Pine martens occur north of Glasgow around Dumbarton and southern Loch Lomond and there are signs they are spreading through the Central Belt, with recent records in North Lanarkshire.

"There are no figures available on whether the pine marten population has increased in number in this area, but the fact that the population is spreading does suggest that the population is increasing in both size and range."

Martens grow to around 30 inches in length if you include the bushy tail.

Their fur is usually light to dark brown and grows longer and silkier during the winter. They have a cream coloured "bib" marking on their throats.

Some people living close to woodland where the martens occur attract them to their gardens at night by putting out food.

The VWT’s work in Scotland has focused on monitoring the recovery and range expansion of the pine marten population.

The survey found that the marten population has continued to expand its range south and east of the Highlands and has re-colonised many parts of its former range in central and eastern Scotland.

One knock-on effect of the pine marten's expansion is that grey squirrels have declined - something welcomed by many naturalists.

The reason is that grey squirrels, which are not a native species, are more dominant than than the native red squirrels and outcompete their smaller cousins which have been in decline.

The larger also greys carry a pox disease which is deadly to reds.

Scientists at the University of Aberdeen found that in places where pine martens were more common, red squirrels were much less likely to visit nut feeders, but greys were not.

Emma Sheehy who led the work said: "This suggested that greys are totally naive to the risks of pine martens as a predator.”

“In their native range, they don’t have similar predators and that leaves them much more susceptible here.”

Mel Tonkin, at the Saving Scotland’s Red Squirrel project. said: “We are very excited to finally have some research which clearly shows that the recovery of the pine marten is having clear benefits for native red squirrels in Scotland.”