Today’s blog is by Conservation Scientist, Connie Tremlett, on her recent field work on the remote island of St Kilda tracking a tiny seabird

The Leach’s storm petrel is one of our smallest seabirds, around the size of a starling. They are beautiful birds – a sleek greyish black, with a domed forehead, a white rump and a forked tail. Unlike many of our other seabirds however, few people are privileged to have close-up views, as they breed on remote islands and spend nearly all their time far out at sea, active at the colony for just a few hours each night under the cover of darkness.

And sadly, the chances of an encounter are lessening – the species has suffered a severe decline and is listed as vulnerable to global extinction by the IUCN Red List.

Leach's storm petrel is now vulnerable to global extinction, but the main drivers aren’t clear (c) Mark Bolton

Answers from the field

But the drivers behind declines are still not well understood, and to better understand what pressures the birds might face while they are feeding out at sea, Marine Scotland commissioned RSPB Science to track them to see where they go.

This brought Mark Bolton, Principal Conservation Scientist, and me to St Kilda this summer, an uninhabited island forty miles off the coast of North Uist, owned by the National Trust for Scotland. St Kilda hosts 94% of the UK population of Leach’s storm petrels, their largest colony in the northeast Atlantic.

We were tasked by Marine Scotland (funded by the European Maritime and Fisheries Fund) to deploy tiny GPS tracking devices on the birds, to give information on where the important feeding grounds are, and where the birds might interact with things like light pollution, fishing, and renewable energy developments. To do that requires catching the birds, all with the relevant licenses and permits.

On St Kilda

So, for over a month, we spent almost every night in a large colony of Leach’s storm petrels. The colony was located on the main island of St Kilda, in a steep boulder field located over an hour’s hilly walk from our base in Village Bay.

Spending the night at a Leach’s storm petrel colony is an amazing experience – the birds are only active at the colony then. They make bizarre, chattering and churring noises that emanate from the air as they whizz round you, and from burrows in the ground where they nest (the sound is described by the Collins bird guide as a pixie chuckling and being sick). Our nocturnal soundscape was further embellished by the eerie cries of Manx shearwaters and occasional cicada-like songs of European storm petrels.

The largest UK colony is found on the St Kilda archipelago, Scotland (c) Mark Bolton

GPS tagging

Our aim was to put our tiny GPS devices on between 10 and 20 Leach’s storm petrels, programming the tags to take a satellite position every 30-60 minutes to give us a detailed map of where the birds go at sea. The small size of the tags means that the batteries don’t last long however, and you can’t download the data remotely. This meant we would need to catch the birds again to remove the tag and get the stored GPS information.

We arrived at St Kilda towards the beginning of the breeding season, when the birds were still laying or incubating eggs; we were lucky to get to St Kilda at all! It is a long and rough boat ride.

Our arrival heralded the start of a bout of bad weather, with strong winds that prevented boats bringing visitors for nearly 2 weeks, and which made fieldwork challenging. Happily we did make it across (albeit with very seasick fellow passengers), and our first job was to check out the field site, find nests, and work out the petrels’ schedules.

Once the egg has been laid, the petrels take turns with their mate to keep the egg incubated, swapping over every few nights for around 40 days. We needed to work out when these changeovers would happen, so we could time deploying our GPS tags on the birds precisely. The GPS tag had to be fitted when the bird was leaving the burrow to go out to sea, to stop the battery on the tag being drained trying to communicate with satellites whilst still underground.

The team spent five weeks working hard to tag the birds (c) Mark Bolton

We then had to predict when the tagged birds would return so we could recatch the birds and get the tags and our precious GPS data back. This was easier said than done!

It was a tough fieldwork season, requiring lots of patience, perseverance, and innovation – by the end of our trip we had scaled 31,000 metres, walked 250 miles, and adapted to a nocturnal routine of leaving for the field site around 7pm and only returning to our beds to sleep at 6am. But the rare opportunity to work close-up with such an amazing bird, and the excitement of recatching tagged birds and subsequently downloading those precious tracks to find out where the birds go, made all the hard work worth it!

Next steps

By the end of our stay, we had managed to successfully track 14 Leach’s storm petrels and collected valuable information on where the birds go while at sea. The next step will be to analyse these data properly, to inform conservation objectives such as identifying the most important marine areas and investigating what threats birds might be facing at sea.

We are proud to present the first EVER track of Leach’s storm petrel from UK

We also placed a different kind of tag on 20 birds which will stay on for a whole year, staying with the bird after they have finished breeding on St Kilda and left for the winter. These tags use information on light levels to give a broad picture of where the bird is in the world (to a few hundred kilometres).

The team will need to find and recatch these birds again in 2022 to get the data, which will give us the first picture of where UK Leach’s storm petrels migrate to for the non-breeding period. Currently no one knows! So keep an eye out for exciting updates on movements of these wonderful birds both in and out of the breeding season!

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