A new open access scientific paper has been published in The Seabird Group’s journal, Seabird, describing surveys of the biggest Leach’s storm petrel colonies in the north-east Atlantic. Conservation Scientist and lead author, Zoe Deakin explains in today's blog.

The Leach’s storm petrel is one of the smallest seabirds in the world – about the size of a starling. It spends almost all its time at sea but comes to land to breed on remote islands in the North Atlantic and North Pacific Oceans.  

A Leach’s storm petrel at Elliðaey Island, Iceland. © Bart Vercruysse/Petrels by Night Project 

The biggest Leach’s storm petrel colonies are in Newfoundland, Canada, and huge population declines have occurred there. The largest colony declined by 42% over 29 years, which represents the loss of over a million breeding pairs. What’s happening in the eastern Atlantic is less clear, but a large decline was detected at St Kilda, the UK’s largest colony, between 2000 and 2006. These population declines mean that Leach’s storm petrel is considered globally vulnerable to extinction. 

Threats 

It’s not always clear why particular colonies are declining, but storm petrels face a range of threats. Their small size makes them very vulnerable to predators and whole colonies have been wiped out by the introduction of mammals such as rats. They’re also vulnerable to predation by larger seabirds such as gulls and skuas. Plus, there’s concern that artificial lighting can attract and disorientate storm petrels, causing them to collide with offshore structures such as oil platforms. On top of this, storm petrels are faced with changing habitats and environmental conditions, as a result of human activity. 

Counting the invisible 

To avoid predators, Leach’s storm petrels nest in burrows or under boulders or vegetation, and they’ll only arrive at or depart from the colony under darkness. This means that during the day you can be standing in the middle of a colony of thousands of birds but have no idea that they’re there! This cryptic behaviour makes them a very difficult species to survey. How do you count something you can’t see? 

  

Leach’s storm petrels nest in burrows or under rocks or vegetation and are only active at the colony at night, making them very difficult to count. © Zoe Deakin 

Luckily, we can count storm petrels using sound. Playing a recording of the Leach’s storm petrel’s distinctive call (described by Collins Bird Guide as ‘a pixie chuckling and being sick’) causes some adults to respond by calling back from the nest site where they’re incubating their large, white egg. By counting these responses and using statistical methods to work out the proportion of birds that respond on each occasion, we can estimate the size of the population, without ever seeing a storm petrel. 

In June 2018 a group of RSPB staff and volunteers travelled to Iceland’s Vestmannaeyar archipelago to help an Icelandic team survey the Leach’s storm petrels of Elliðaey Island. The results of a previous survey in 1991 suggested that the Vestmannaeyar archipelago held the largest population of Leach’s storm petrels in the eastern Atlantic.  

Then, in June and July 2019 a team of fieldworkers conducted similar surveys on the four main islands of the remote St Kilda archipelago, in Scotland’s Western Isles. Some of St Kilda’s islands are incredibly difficult to land on and are very rarely visited. The last survey of the whole archipelago was in 1999-2000.  

What did we find? 

Unfortunately, it wasn’t good news. Our results suggest that Elliðaey’s Leach’s storm petrel population currently stands at around 5,400 pairs, having declined by 29% in 26 years. On St Kilda, the decline was even more severe: a loss of 68% in 20 years, with a current population of around 8,900 pairs. Since St Kilda previously held 94% of the UK and Ireland’s Leach’s storm petrels, this large decline led to the species being classified as Critically Endangered in the UK in the recent Birds of Conservation Concern report. 

 

Leach’s storm petrels nest on remote islands such as Elliðaey Island in Iceland (top) and Boreray, part of the St Kilda archipelago in Scotland (bottom). Recent surveys suggest the Leach’s storm petrel populations on Elliðaey and St Kilda have declined by 29% in 26 years and 68% in 20 years, respectively. © Zoe Deakin 

Next steps 

We still don’t fully understand the causes of these declines, but there are likely to be multiple factors involved. It’s crucial that we understand the movements of Leach’s storm petrels at sea, since that’s where they spend most of their time. In 2021 Marine Scotland commissioned the RSPB to track the species from St Kilda for the first time and this work will continue in 2022. The results have already improved our knowledge of Leach’s storm petrels in the UK and will hopefully go on to inform conservation efforts and marine management decisions that could impact these elusive and threatened little seabirds. They’re going to need all the help they can get. 

Funding 

The Leach’s petrel survey of the St Kilda archipelago is part of ‘Seabirds Count’ (2015-21), the fourth national census of the UK’s breeding seabird populations. It was made possible using funding received by RSPB and JNCC from EDF Renewables, Moray Offshore Windfarm (West) Limited (Moray West), Red Rock Power Limited (Red Rock Power) and SSE Renewables.

For the full paper in Seabird: Decline of Leach’s Storm Petrels Hydrobates leucorhous at the largest colonies in the northeast Atlantic

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