Missing Terns

Any recent visitor to Dungeness will have noted the absence of common terns on Burrowes Pit and all around the reserve this year. This is very disappointing after several good years in a row for breeding terns until last year, so we have done a little digging into what may have happened.

Dungeness is not alone in this situation, Wallasea Island are reporting far fewer numbers of terns after years of increase like Dungeness and there are other sites too.

It is important to understand that it is entirely natural for seabird colonies to be mobile, therefore it is always ideal to have a series of potential colony spots within a small area to allow them to move and always have nesting habitat available. For example, the Solent has a lot of small colonies of seabirds on islands within the harbours and thanks to the hard work of the teams down there, there are now lots of potential nest sites for the terns. Dungeness has Rye Harbour within close flying distance and there are nesting colonies on the French coast and it is perfectly normal for the terns to move between these areas.

The graph below shows the natural movement of tern colonies between two sites, as you can clearly see the movement from one site to the other.

So, it would be fair to assume that if they aren’t here, then they would be at Rye or the French coast. However, conversations with the teams there have shown no big increase in the number of terns at either site with an additional 30 pairs at Rye and similar numbers to previous years in France and around the Solent. This doesn’t account for the 60-70 pairs we were expecting, so the population is just simply not around.

What could have happened

Common tern breeding numbers had been very good since the Re-Tern Project was completed in 2017 until last year. Productivity was very impressive as show in the table below:

Asterix indicates underestimate due to lack of monitoring time due to pandemic

In 2021, we experienced a very wet and cold June which unfortunately coincided with the first brood of hatching chicks who didn’t stand a chance with those conditions. This weather persisted for so long, the terns just gave up and didn’t attempt a second brood. More extreme weather can be expected as part of climate change, so years like 2021 have to be expected more often, but it doesn’t seem to be the reasoning for the terns not breeding this year. The weather was cool again this year, but there were not the large numbers of terns overhead to even evaluate the breeding conditions.

We have had low levels of chick predation in recent years by gulls, but clearly not to the detriment of the population as a whole as can be seen in the table above. Low levels of predation are simply part and parcel of a seabird colony and that is what we are seeing on the reserve.

Breeding habitat is present thanks to the Re-Tern Project, the islands have all been raised and thanks to a dry spring they have all been exposed at the right time and there are plenty of islands free from gulls. We can see minor levels of erosion on some of the islands over the past 5 years, but there is little that can be done about this due to the depth of the water around the islands (7m deep!), as has been mentioned in previous blogs, we do not have any control on water level as the reserve sits on a ground water aquifer. Island space is therefore not a reason for the loss of the colony either.

Although we haven’t had a fish survey of Burrowes completed recently, there seems enough evidence out there in the shape of gulls, herons, cormorants and kingfishers amongst other species to assume that fish supply is not an issue in the lakes on the reserve. We are even seeing a few terns on site fishing the lakes, probably breeding pairs from Rye. Whether the power station hot water outflow has had an impact on feeding (it was a great source of food for many seabirds as fish were attracted to the hot water) we aren’t sure. It is a potential that there simply aren’t enough small fish in the sea in the area anymore.

This brings us to a newer issue which has been reported on seabird colonies around the UK and France and has to be considered strongly as to why the terns haven’t returned - Avian Influenza (AI). AI has hit bird populations around the UK very hard and lots of seabird colonies are reporting high mortality rates. Some of the North French coast colonies are reporting losses of terns due to AI as well, so its very close to home. Terns are often in very close proximity to each other so it would be easy to pass any AI around large numbers.
So whilst we can be confident to rule out some reasons for the decline, there is still further work to be done.

What’s next

We know the terns have recovered from a 0 nest season before – as recently as 2008. In fact between 2005 and 2011, only 13 terns nested at Dungeness in that entire time period and we have recovered to the good numbers seen post Re-Tern Project.

We can only hope that they do return and that AI does not continue to have the large impact it appears to be having on many species of birds in the UK.

Whilst some things such as food supply, the impacts of climate change and AI are out of the reserves control we can ensure the habitat continues to be in excellent condition should the tern colonies re-appear in the future. We still hope to undertake the island raising on ARC this autumn if the water levels continue to facilitate this and we will be starting work on the seabird raft repair at ARC in the coming weeks too.

For further information or if you have questions, please don’t hesitate to contact the team.

Common tern image courtesy of Graham Parry