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.
Guest Blog by Sarah Dalrymple South Walney Manager & Reserves Officer, Cumbria Wildlife Trust
The gull colony at South Walney is an incredible one, and its fortunes over the years mirror strongly that experienced by the UK’s two commonest gulls, the herring and lesser black-backed gulls.
Although a few gulls nested in the early 1900s, the numbers were steadily increasing. With the South end of Walney Island established as a Nature Reserve in 1963, regular and reliable counts were undertaken, and by 1974 an incredible 41,366 pairs of gulls – 22,751 herring and 18,615 lesser black-backed – were counted, making this largest colony in Europe at this time.
Speaking to older birders who remember the height of the colony, a daily job at this time was to walk the tide line at low tide and collect the corpses of hundreds of gulls that had died from botulism over the previous day. The main food source for the colony was presumed to be a landfill 2.5km up the road from the colony; as this reduced in scale over the years until complete closure in the 1980s, the colony size declined accordingly.
Remaining stable at about 10,000 pairs for a few years, the colony then started to suffer from mammalian ground predators. Remarkably, as Walney is only separated from the mainland by a narrow channel, foxes were not present on the south end of the island until the 1990s, and badgers not established until the late 2000’s.
In 2011 Cumbria Wildlife Trust trialled temporary electric fencing of a part of the colony, which saw a notable success in fledging compared to the rest of the colony. Two large areas of the remaining colony were fenced – The Spit, a shingle area at the very southern edge of the reserve, and Gull Meadow, an area of semi-fixed dune on the western shoreline of the reserve.
Although initially successful, a new problem arose in the mid 2010s. Year on year, several thousand pairs were fledging zero chicks. The most harrowing was in 2017, as we found a busy colony full of gulls and their young was carpeted with dead chicks only a week and a half later. This scene was repeated the following year.
Necropsies were carried out by the Animal & Plant Health Agency, and it was identified that most chicks had been killed not by predators, but with “peck” marks. It was highly likely that the gulls were, in fact, killing their own young, or the young of their neighbours, and then not eaten – just left where they were.
This is not an unknown phenomenon, as we’d seen this at Rockcliffe Marsh gullery, another site managed by Cumbria Wildlife Trust, and it has been seen at Gulleries in Texel, the Netherlands – although not to the same extent. We think that this intra-species predation is caused by the gulls experiencing extreme stress, loss of a food source plus predation pressure plus disturbance all adding up to cause the gulls to kill either their own or their neighbour’s young.
By removing predation and disturbance from the Rockcliffe Colony, we managed to stabilise it after several years of zero productivity, and it is now a recovering colony successfully fledging young each year.
At South Walney, a landfill just over the bay had closed in 2016, GPS tracking from a BTO project suggested our gulls had been feeding there; this closure led to a huge food source loss. And our electric fence was only mostly keeping the predators out; badgers and foxes were still sometimes getting through, but in 2020 badgers not only happily pushed through the fence, they dug a temporary sett right in the middle of the colony!
By this point South Walney had dropped to under 1,000 pairs. Clearly something drastic was required to save it, and at least by removing any risk of predation we’d give the gulls the best chance we could. So thanks to our funders, this winter we erected a permanent fence, nearly 2m high, topped with electric, and with a horizontal “skirt” extending out underground 50cm to prevent any digging underneath. We enclosed almost the entirety of the Spit with this fence, double the area of the original electric fence.
The eggs have started to hatch, and our counts reveal a new record low of just under 500 pairs. Will the new fence be enough to save the gull colony?
Photo credits: Oystercatcher by Katie Nethercoat (rspb-images.com)