Gannet numbers off the northwest coast of Scotland have increased rapidly over the last decade, a study has found.

It follows news last month that the Bass Rock in East Lothian took over from St Kilda as the world's largest gannet colony. The latest results confirm that Scotland.

Scottish Natural Heritage (SNH) commissioned the survey which found that gannets nesting at Sule Skerry, 40 miles west of Orkney increased at a rate of more than 47% per year. On the Flannan Islands, 20 miles west of Lewis, the growth was 7.5% per year. Numbers changed very little on St Kilda, now the world's second-largest gannet colony, and Sule Stack near Sule Skerry.

Gannets are Scotland's largest seabird, weighing between 2.5 to 3.5kg, with nearly a six-foot wingspan. They migrate to the west coast of Africa, and sometimes further, in the winter. They only lay one egg and pairs typically mate for several seasons, if not for life.

The SNH study says the good health of gannets across Scotland might be explained by their foraging behaviour. They can fly for great distances in search of food, and are able to take a wide range of prey species. They also usually nest at sites which have few predators, have produced lots of chicks over the past decade, and have high survival rates.

Eileen Stuart, SNH's head of policy and advice, said:

"Scotland has internationally important populations of seabirds, and gannets in particular, so it's vital that we monitor how they're faring. The survey is especially good news as gannets are, like many seabirds, of conservation concern."