The population of Europe’s smallest seabird, the storm petrel, doubled at Mousa in just 12 years, according to new figures released by RSPB Scotland.
A survey conducted in 2008 by RSPB Scotland and Scottish Natural Heritage recorded an estimated 11,800 pairs – a huge increase on the 5,400 found in 1996.
As well as nesting in drystone walls and boulder beaches in Mousa, storm petrels famously nest in cavities in the Iron Age broch to which they return to under the cover of darkness. This nocturnal behaviour means that they are notoriously difficult to count.
To monitor the population, researchers used a technique that involved playing a tape recording of a storm petrel call into the potential nest sites. Every return call was counted, which demonstrated a substantial increase in the storm petrel population between 1996 and 2008.
Mousa has nearly 40 per cent of the UK population of storm petrels, making it the largest colony of the tiny seabirds in the country.
Glen Tyler of Scottish Natural Heritage said “We’re delighted with the results of this survey. One of the keys to the success of this colony is the absence of introduced ground predators on the island.
“Rats, mink and feral cats all prey on storm petrels and the presence of such predators has led to the disappearance of some colonies in the UK. It is essential that Mousa remains free of ground predators if we are to safeguard the long-term future of this species.”
The increase of the population in Mousa is particularly noteworthy against the background of widespread declines of other seabird species in the UK.
RSPB Principal Conservation Scientist Mark Bolton believed that could be down to a number of factors. He said: “Despite being a similar size to a sparrow, storm petrels are capable of flying hundreds of miles from the colony to search for food, and chicks can tolerate periods of several days without being fed by their parents.
“They feed on a wide variety of prey species, which makes them resilient to the declines in any one particular food type. The fortunes of many other seabird species are closely tied to sandeels, whose stocks have diminished in recent years.”
Being at the top of the food chain, studying seabirds added to people’s knowledge about the marine environment, Dr Bolton said.
“Long-term monitoring is essential to help us gain a better understanding of the health of the seas around us,” he added.