Storm petrel colony doubles in size

Nesting cavities
As well as nesting in dry-stone walls and boulder beaches on 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 this 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.

40% of UK population of storm petrels
Mousa has nearly 40% of the UK population of storm petrels, making it the largest colony of these tiny seabirds in the country. Glen Tyler of Scottish Natural Heritage, who joined RSPB Scotland in conducting the survey, 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."

Most seabirds in decline
The increase of the storm petrel population on Mousa is particularly noteworthy against the background of widespread declines of other seabird species in the UK. Dr Mark Bolton, RSPB Principal Conservation Scientist says this could be down to a number of factors: "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 sand eels, whose stocks have diminished in recent years".

"Being at the top of the food chain, studying seabirds adds to our knowledge about the marine environment," added Dr Bolton. "Long-term monitoring is essential to help us gain a better understanding of the health of the seas around us."