Blog by Mhairi Maclauchlan, RSPB Cumbria Coast Reserves Warden
Hello, I’m the Warden for the Cumbria Coast Reserves and I’ve been asked to give a summary of our long term tern numbers at Hodbarrow. Hodbarrow is a large lagoon in the south of Cumbria formed from a former iron ore mine. It’s calm waters and iron ore slag islands provide ideal habitat for nesting birds in particular seabirds such as terns, ringed plover, oystercatcher amongst many more.
Previously my colleagues have written about the work being carried out at Hodbarrow over the last year thanks to EU Life on The Edge funding. In addition to what is happening presently it’s very important for to look at historic trends in the data, examine the reasons for those trends and use these to help shape our management. Using historic data as well as data collected by the dedicated wardens we have had in place over the last 5 years, we can see how numbers have changed. When looking at data we tend to look at population size and productivity.
Numbers of nests only give us a small glimpse into the breeding season it can also be useful to investigate the productivity of those nests. When we say productivity, we mean the success of each individual nest to raise young to fledging age and the numbers of fledglings coming from each nest. Fledging survival to breeding age isn’t guaranteed but we still think of that nest as productive. Numbers of nests, in simple terms, show us what birds arrived to breed on the habitat we have however productivity shows us how many chicks were produced from those nests that will now be part of the population.
Focusing on Hodbarrow you can see from the graph below that Sandwich terns would arrive and not progress to nesting or fledging stage and have shocking productivity.
After several years of this trend in the winter of 2015/16, we installed an in-water fence to stop predators such as fox accessing the island and decimating the nesting birds. This was also the start of our employment of summer wardens who were responsible for managing disturbance and large gull predation. As you can see after these measures were implemented birds were able to settle and breed as well as successfully get chicks fledged.
South Cumbria Populations
It can be very tempting to focus in on your site and what is happening locally, when in reality birds don’t see a boundary or a fence on the map. A few years ago, we came to the realisation that we had to look at Hodbarrow numbers (successes and failures as well) in terms of South Cumbria, national and even international populations rather than focus on purely Hodbarrow numbers. This gave us a wider and better understanding of what was driving our population changes.
There are two main tern colonies in South Cumbria – Hodbarrow and Foulney. They are geographically very close to each other as the tern flies.
We work closely with the Wildlife Trust who manage Foulney, sit on working groups with them and we have also worked together throughout Life on The Edge funding.
We see the birds at Hodbarrow as very much part of the South Cumbria population. Often the overall number in the population of birds in South Cumbria stay the same however they utilise both sites. One year they can all be at Hodbarrow and another year the population can then move over to Foulney.
The graph below shows the population changes for little terns over 30+ years. You can see from the graph that from 1996 – 2003 little tern numbers were concentrated at Hodbarrow and similarly from 2008-2012 Foulney held most of the birds, however overall, the population number stays roughly the same.
The trends in the Cumbrian population shows that even though there are years where birds aren’t present on a specific site, we still have to make sure the habitat is great and that work carries on to provide tip top potential nest site which they may utilise during the season or in subsequent years.
Interestingly, we have even seen this trend with colonies further afield. In 2018, we had the best ever year for Sandwich terns at Hodbarrow / South Cumbria however this involved birds re-locating from Cemlyn, Wales. Within days of a mass desertion at Cemlyn due to increasing otter predation a large influx of birds arrived at Hodbarrow. We can only surmise that these birds may have made the journey up the coast and the dates certainly provided evidence in favour of this.
Guest Blog by Chris Goding, Hodbarrow Field Officer
Following Dave Blackledge’s introduction to the recent LOTE funded habitat works at RSPB Hodbarrow, life at the reserve continues apace. I am one of two Field Officers here over the breeding season, with shared responsibility for surveying adult numbers and productivity of our key species (notably Sandwich, common, and little terns). We also monitor predation events and engage with members of the public about the RSPB’s work at the reserve, its history, and the wildlife found here.
Work was finished on the new island in January this year, complemented by an extension to the eastern side of the main island. The new island has seen modest but encouraging interest from breeding birds, and is currently home to a ringed plover pair (with two chicks) and a single oystercatcher nest as well as 2 common tern nests.
We are hopeful of increased use of the island in future years once the substrate matures. At the time of writing this blog, there are at least 20 common tern pairs on the new extension to the main island, making use of the increased space provided where a small island has been joined to the ‘mainland’, forming a miniature peninsula.
Current trends point to a successful season all round!
The peak count of little tern adults so far this season is 87, with at least 40 pairs, more than triple the number of pairs last year! Twenty six little tern chicks were spotted during a ringing session on the 11th June, the highest count since at least 2017, so we are hopeful of an excellent year for the species here. They appear to be responding particularly well to the application of slag to the concrete surface at their favoured spot, which helps to prevent the accumulation of rainwater around the nests.
Fifty common tern pairs (with a minimum of 40 chicks so far) points to a good year for this species too. Alongside this, at least 300 black headed gull chicks and 200 Sandwich tern chicks means the colony is a busy place.
With time yet for these numbers to increase we are expecting a productive season at Hodbarrow this year.
By Dave Blackledge, RSPB Site Manager - Cumbria Coastal Reserves
RSPB Hodbarrow sits on the edge of the Duddon Estuary in south-west Cumbria. Part of the Morecambe Bay and Duddon Estuary SPA, it was the site of one of Europe’s most productive iron mines in the 19th Century.
As mining operations ceased in 1968, a mixed colony of Sandwich, common and little terns began to nest on the limestone slag by the lagoon formed on the flooded workings. This colony is now of international importance and its position behind the sea wall, protecting the site from storms, high tides and future sea level rise mean it is an integral part of western Europe’s tern network.
Introduction of anti-predator fencing in 2016 immediately reversed the fortunes of the colony after a few years of fox predation, with tern numbers increasing rapidly. In 2018, predator disturbance of the Sandwich tern colony at Cemlyn on Anglesey led to a large influx at Hodbarrow with 1950 pairs, around 15% of the UK population breeding here that year. The fox free breeding islands have also benefitted other shorebirds, with black-headed gulls, eider, tufted duck, ringed plover and oystercatcher all benefitting and increasing in number.
With all this activity concentrated on a single 1.5ha island it became apparent that nesting space was becoming an issue. Little terns in particular, arriving and settling later than many of the other species were being squeezed to the edges of the island, picking sub-optimal nesting sites wherever they were able to find their preferred semi-isolated spot.
Life On The Edge is addressing this problem by creating further protected breeding habitat at Hodbarrow. Around 12500 tonnes of limestone slag from the adjacent slag bank will be dug and transported to increase the size of the current breeding island from 1.5 ha to 2 ha, alongside the creation of a second 0.25ha island.
A further 130m of fencing to protect the new island will also mean that breaching of defences to one island will not give foxes access to the entire breeding area, giving a further level of protection.
Other works include scraping of vegetation and scrub from an artificial flood bank left over from the mining operations. The aim is to provide further nesting opportunities for the SPA designated lesser black-backed gulls along with herring gulls and great black-backed gulls.
The main habitat works on site have progressed well this winter (while complying with the covid-19 guidance) and we now eagerly await the 2021 breeding season to see how the birds respond to the improved and new islands.
Photo credits: Oystercatcher by Katie Nethercoat (rspb-images.com)