St Kilda's Leach's storm petrel chicks shed light on how a rare seabird is faring

One of St Kilda's Leach's storm petrel chicks

A handful of little balls of fluff hold the answers to the health of the UK's largest breeding population of a rare seabird.

Leach's storm petrels have declined in numbers globally.

Scotland's remote St Kilda archipelago is home to 94% of the UK's breeding birds.

Conservationists have been weighing and measuring five of St Kilda's chicks on a daily basis to better understand how the wider population is faring.

The scientists are able to check on the chicks while the adult birds are at sea for most of the day catching food.

A handful of St Kilda's are weighed and measured daily until they fledge

Leach's petrels nest in burrows but five chicks - with a sixth also to be weighed and measured once it is old enough - are being raised by their parents in one of 48 nest boxes.

The five have been given names by the St Kilda Ranger service following suggestions from the public.

They are Ceòl Mòr, Ceò, Luath, Sweep and Pebble.

St Kilda has 94% of the UK's breeding population of Leach's petrels

On hatching Leach's petrel chicks weigh less than 10 grams.

The daily health checks involve measurements of the birds' weight, wing length, head-bill and part of the foot called the tarsus.

Seabird and marine ranger Sarah Lawrence said: "A chick's weight can fluctuate dramatically from day to day as they 'yo-yo' towards a peak weight of 70-80 grams while they grow, before dropping to 55-60 grams allowing the chick to fledge the nest.

"The chicks will finally fledge in September-October at between 60 to 70 days old."

The chicks should reach a peak weight of about 80 grams

She added: Leach's petrels have been declining at their nest sites on both sides of the Atlantic, and St Kilda hosts 94% of the UK's breeding population.

"It is hoped the nest boxes on St Kilda will be able to facilitate further research and help us better understand possible causes of population decline."

This year a full population survey of the Leach's storm petrels and other seabirds nesting across the St Kilda archipelago was completed for the first time in 20 years.

The work was led by Dr Richard Luxmoore, of the National Trust for Scotland (NTS) with a team of researchers from the RSPB, University of Cardiff and University of Gloucestershire.

The results of this survey are expected later this year.

Adult Leach's petrels spend most of the day hunting for food at sea

St Kilda lies about 40 miles (64km) west of North Uist, the nearest inhabited place to the archipelago.

The last islanders left St Kilda in 1930 and people only now live on main island of Hirta on a temporary basis to work at a military radar site, or on wildlife conservation projects.

NTS manages the islands as a Unesco World Heritage site.