Treshnish Storm Petrel Census 2019

At the end of a long and busy field season seabird researchers love nothing more than… more seabird work for their holidays.

That’s how two members of SEGUL, Lila Buckingham and Sophie Bennett, spent a week this July while volunteering with the census of the Storm Petrel population breeding on the Treshnish Isles.

Here, Sophie Bennett tell us about their trip and why the survey was going ahead.

The Treshnish isles are a group of islands just off the west coast of Mull in the inner Hebrides. The Treshnish Isles Auk Ringing Group (TIARG) has been monitoring the seabird populations breeding on this group of islands since 1971. Every year the team returns to census the breeding birds, fit adults and chicks with rings engraved with a unique identifier, and in recent years have also been tracking the movements of adult seabirds over winter; the islands also form one of Lila’s study sites for her PhD.

I felt hugely privileged to be able to join the team this year to help census the breeding population of European Storm Petrels, Hydrobates pelagicus, a species less familiar to myself as in the summer I am normally found on my PhD study site, the Isle of May, on the east coast of Scotland. Stormies, as they are affectionately known, are swallow-sized burrow/crevice-nesting seabirds in the order Procellariform which also contains their larger cousins, Albatrosses, Shearwaters and Fulmars. These small seabirds, the tiniest of all the petrels, is the most common storm petrel in the UK, which was estimated at ~24,000 pairs in 2000. These small birds may migrate well south of the equator during the winter months, a period in which they are entirely pelagic and won’t return to land again until they breed the following spring.

A Stormie incubating its egg, seen on the Treshnish Isles in 2018 ©Daniel Plunkett

The TIARG team began their census of the Stormies in 2018 as part of the UK Seabirds Count. The seabird count is a national survey carried our ~every 15-20 years across the UK, where the population size of all breeding seabird species in the British Isles are quantified. The next census is currently being completed which means that we should soon have an updated understanding of the current status of all UK seabirds.

As the Treshnish Isles are a group of eight large islands and numerous smaller islands and skerries with often rough terrain, surveying them all is a huge undertaking, and so while the TIARG team began surveying last year, there were four islands remaining to be surveyed this season: Cairn na Burgh Beag, and Cairn na Burgh Mòr in the north of the island chain, and the two ‘Dutchman’s’ islands, Bac Beag and Bac Mòr in the south.

The Treshnish Isles ©Southern Hebrides

And, so, after spending the night on Mull at Tim (our glorious leader) and Helen’s house, we made the crossing to Lunga, the island that would be our home for the next week.

As an island with no permanent residents, an old Scottish Blackhouse/ Bothy served as our living quarters, which we made homely by adorning it with candles, and ‘seabird-themed’ tea towels which unintentionally and inevitably were all ‘puffin-themed’.

Images clockwise: Our Bothy mansion ©Daniel Plunkett, homely adornments ©Lila Buckingham and a cracking view to wake up to ©Chris Heward.

With our camp set up and dinner underway we sat down to discuss our plan for the week and to go over the survey methodology.

Storm petrel populations can’t be monitored very easily visually as they nest underground in narrow burrows and crevices that can be over a metre deep. However, stormies will respond to a recording of their call being played near their burrow and call back with their bizarre gremlin-like call that sounds like ‘Turr-chickk’, or they may make a ‘purring’ sounds. (Noise recording ©Andrew Carter.)

As a result of this, the main monitoring method used is to play their call for the same length of time at points across a transect in all ‘suitable’ habitat, e.g. old stone walls, buildings (including our bothy!), and boulder scree, and record the number and type of calls that we hear in response. As not all individuals will always respond to recordings we would also set up calibration plots on Lunga that we would survey every day to work out an approximate response rate to compare our actual survey data with. From these two sets of data we could then calculate the apparent number of breeding pairs.

Lila and Chris listening hard for a Storm Petrel ‘Turr-chickking’ ©Matt Williams

Now this methodology sounds simple and straightforward, which it is, however unfortunately the weather was very much against us for the first five days of our trip which prevented us from being able to get to the islands remaining to be surveyed. While frustrating in terms of completing the survey this time did allow us to explore the island, resight European shag colour-rings as part of the long-term seabird study on Treshnish, and for our more creative members of the team, to upcycle the camp’s old toilet seat into the ancient Mesopotamian game of Ur which kept us very happily entertained. What do you mean you haven’t heard of it?

Left: A tense game of Ur played during a downpour ©Lila Buckingham, right: our fantastic board, dice and shell pieces made by Chris and Lila ©Sophie Bennett

Just as we were beginning to think that we might not make it to any of the islands we needed to survey, Turus Mara, a tour boat company on Mull that operates tourist trips to Lunga and facilitates all of the monitoring work done by TIARG, rang to say that the weather looked likely to clear enough to allow us to get to the largest of the two Cairn na burgh islands.

Finally on the way to census Cairn na Burgh Mòr ©Lila Buckingham

Relieved at the news, we organised our equipment and boarded the boat. We arrived on the island keen to get going and completed our survey of the larger of the two islands, where the majority of birds were believed to breed. In between surveying we had a small amount of time to explore the Iron Age fort and walls on the island, which we found had been made the homes of many storm petrels. But all too soon our time was up and we were picked up again and returned to Lunga, saying goodbye one member of our team, Matt Williams, who had to return to mainland duties.

Left: An unusually easy-to-see nesting Storm Petrel ©Sophie Bennett, and right: the view from an Iron-Age fort on Cairn na Burgh Mòr ©Sophie Bennett

Pleased with our success and relieved that we were able to further the census we retired to the bothy for the evening, longingly eyeing the Dutchman islands on the horizon.

Tim making an offering to the weather gods ©Chris Heward

On the morning that we thought would be our last in the island we received another call from Turus Mara who said that they thought that we may be able to get onto the Dutchman’s islands if we stayed one more day. Keen to complete the survey we agreed to stay and try our luck the next morning, after receiving fresh supplies of bread, milk, and most importantly whisky.

The next day we rose early to move our equipment and supplies all the way back down the hill to the boat landing and awaited our chance of getting to our final islands. On cue the heavens opened and we got set in for what was to be an incredible day of rain. Shortly, Turus Mara arrived to pick us up from Lunga and we set off to chance a landing. Miraculously when we reached the islands the swell had subsided, and so at long last we made it to the Dutchman’s.

Soggy, yet victorious, we had arrived on the Dutchman’s ©Lila Buckingham

A very different island to the other’s we had been on this week, the Dutchman’s were much less visited, with only two landings by TIARG since 1973. Presumably as a result of this reduced visitation there were rich banks of wildflowers carpeting the islands and a great many pairs of great skuas, black guillemots and puffins keeping a watchful eye on us.

As before, we had just enough time to survey the islands and inhale our lunch before it was again time to go. And so, weary and in need of a warm shower we were picked up by Turus Mara and taken back to Mull for another night at Tim and Helen’s house and a much-deserved meal out to celebrate our stormie success.

Tim, the king of Dutchman’s ©Chris Heward

While we strongly suspect that the stormie population on Treshnish has increased and expanded since the last census, there is a still some final calculating to do to reach a final answer as to exactly how the population has changed on the islands. But happily it is likely that the outlook is very positive for the stormies; you can keep up-to-date with the results of the census on the TIARG website.

Our excellent team for the week, pictured left to right: Chris Heward, Lila Buckingham, Daniel Plunkett, Helen Stace, Sophie Bennett, Tim Dixon, Jamie Dixon., and not pictured here, Matt Williams. ©Daniel Plunkett

I really enjoyed being able to see and experience the Treshnish Isles and having the opportunity to survey a new seabird species. I couldn’t have asked for a better team of folk to work with and I know I’ll be missing the sounds of Manx shearwaters calling as I fall asleep for a long time to come. A huge thanks again to Turus Mara for making all of our work possible, and to the owners of the isles, The Hebridean Trust, for allowing us to carry out the survey and to Scottish Natural Heritage for helping to fund to survey.

See you all again for the next survey in 2040!......

Sophie Bennett

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