Blog by Dr Mark Bolton, Principal Conservation Scientist on his new paper published today. 

The sparrow-sized European storm petrel is the smallest seabird in the Atlantic, nesting only on the most remote islands, around the coasts of north-west Europe. Over the last century, seabird biologists working in these far flung and beautiful places have pieced together the details of the storm petrels’ breeding behaviour on land. We know that they only visit the nesting colony under the cover of darkness, and parents take turns to incubate their single egg for periods of up to three or four days, without feeding, whilst their partners remain at sea.

However, until now, our understanding of their activities at sea was based largely on guesswork. We knew from boat surveys that the highest densities occurred along the edge of the continental shelf, bordering the open ocean. We knew little about the comings and goings of these tiny birds between the foraging areas and the nesting islands.

Storm petrel at sea (c) Mark Bolton

The recent development of miniature GPS tracking tags has provided researchers with a remarkable new tool that enables us to accompany these tiny birds on their foraging trips. It allows us to understand for the first time, their movements, behaviour and distribution at sea.  The information we can now obtain provides fascinating insights into the birds’ daily lives. This will produce vital evidence to ensure that important areas for feeding and commuting are well-managed, and ensure the continued survival of these remarkable seabirds.

The study site and tracking work

The work was carried out on the RSPB reserve of Mousa in the Shetland Islands, which is home to around 11,000 pairs of storm petrels, the largest breeding colony in UK, and around 2% of the global population. The birds nest in crevices in dry stone walls, boulder beaches and ruined buildings. Storm petrels have been studied on Mousa for nearly 30 years, and the nesting areas and individual pairs are well-known, enabling the birds to be carefully handled to attach and remove tags with the minimum of disturbance. Over the course of four years birds were tracked during different stages of the breeding cycle – when they had eggs, small chicks, and later in the season when the chicks were larger.

Storm petrel chick (c) Mark Bolton

Where did the birds go?

I had anticipated that the birds would roam far and wide during the course of their foraging trips, and most would head out westwards to forage at the edge of the continental shelf where high densities of birds have been reported. I was amazed when downloading the data from the first bird tracked, and saw that it had headed to the south of the Shetland Islands, feeding far from the open ocean in relatively shallow waters to the south-east of Orkney.

I thought perhaps the next birds would head west, but as more birds were tracked the same pattern was repeated and over the course of the study a high consistent picture emerged. Most birds were foraging to the south of the Shetland Islands, up to a range of about 300km from the colony. An earlier review of the foraging ranges of all UK seabirds suggested that their range was “more than 65 km”, but was unable to say how much more than 65km it might be!

Since the tags tracked the birds 24/7 we were also able to follow their movements back and forth from the colony under the cover of darkness, revealing high densities of birds in the waters surrounding the colony after nightfall. The highly consistent usage of feeding areas south of the colony and the commuting areas around the colony, coupled with the very large number of birds breeding at the colony, means that both these two newly discovered sites comfortably exceed the criteria for identification as “marine Important Bird Areas”.

Storm petrel with a GPS tag (c) Mark Bolton

The tracks of particular individuals also provided a fascinating window into the conditions the birds have to deal with on a regular basis. One bird tracked during a period of strong westerly winds was blown right across the North Sea to the coast of Norway, north of Stavanger, and took refuge, during daylight, in a fiord, before heading back to Shetland the following day. Just 48 hours after leaving its nest it was safely back home again, having travelled nearly 1000km, and when I weighed its chick, discovered it had been fed! Unless the bird had been equipped with a tag we would never have known about this remarkable journey, and yet it’s likely that all storm petrels out feeding on that day would have had similar stories to tell.  

The next steps

The data collected so far have shown us where the birds are feeding, but not why they are using these particular areas. The next phase of research will focus on identifying the birds’ prey, using DNA analysis. Once we know what the birds are feeding on, we will then examine the distribution of the prey itself to see whether that might explain the highly consistent pattern of storm petrel distribution, or whether other factors might be responsible. Watch this space…

For the full paper in Bird Conservation international: GPS tracking reveals highly consistent use of restricted foraging areas by European Storm-petrels Hydrobates pelagicus breeding at the largest UK colony: implications for conservation management.

As part of this study we spent many weeks on Mousa, over several years. In 2015, one of our volunteers was photographer Josh Murfitt, who spent ten days with us on the island helping out with the storm petrels. Josh has recently created an online exhibition of photos he took of the study sites and fieldwork which you can see here.

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