Mid-season update from the Dublin Port Tern colony

The Tern season is in full swing, and Dublin Port, with four structures set aside for nesting Terns, is alive with the sounds and sights of Common and Arctic Terns busily raising the next generation.

Terns over ESB platform in Dublin Port
Photo by Tara Adcock

So far this season we have carried out two nest censuses of the Arctic and Common Tern colony breeding in Dublin Port. Numbers have increased significantly over the last 35 years, going from 38 pairs in 1984 to nearly 600 nests last season.

In total, four sub-colonies of Arctic and Common Terns nest in the port on man–made structures, (see this and this blog post for a background to the Dublin Port Tern colony and DBBP’s work). These have been re-purposed specially for breeding Terns, with gravel added as a nesting substrate, and chick shelters placed on each. Our job is to make our way to the breeding structures under the able navigation of Jimmy Murray and colleagues from Poolbeg Yacht Club, and count the number of nests, eggs per nest, and chicks. As of the last census, we have counted nearly 600 nests this season, which, all going well, means it looks like we are on track for another successful breeding season on most breeding platforms in the port.

Jimmy Murray, our able captain. The black pipes on the platform act as chick shelters.
Photos by Tara Adcock.

Nest censusing.
Photo by Tara Adcock.

Counting Tern nests sounds like a straight-forward task, and it is, once you get the eye in. However, when confronted with your first nest census, it can feel a bit like a game of Where’s Waldo. Terns don’t construct nests in the sense that we are traditionally familiar with. Instead, they make a small hollow or scrape in the ground and lay the eggs in this. To make it a little more difficult for potential predators, the eggs and chicks are well camouflaged, using dark blotches against a lighter background. This technique is known as disruptive colouration and works by breaking up the outline of an object (Stoddard et al. 2016).

Two Tern chicks and one egg.
Photo by Helen Boland.

Two Tern chicks
(one newly hatched and still wet).
Photo by Helen Boland.

To add an element of danger to the whole process, Common and Arctic Terns are fiercely protective of their offspring, often dive-bombing us as we move about the platforms. Terns subscribe to the philosophy of ‘strength in numbers’. They typically prefer to nest in large colonies, where they practice what’s called ‘group mobbing behaviour’, working as a team to drive intruders away from the breeding site (Mallory 2016). Thanks to their ability to work together, increasing numbers within each sub colony will go some way to protect these Terns from potential predators. (See below for a video of the Dublin Bay Bird's Project, Ricky Whelan, being mobbed by a very persistent Tern!).

Once the nest census is complete, it is off to the hide - supplied by Dublin Port Company earlier this year on one of the platforms - for some Tern ring -reading. The hide is a wooden structure, with three windows that allows us to view the Terns, while limiting disturbance to them as a result of our presence. We’ve also found that it’s relatively roomy, and can squeeze in three adults, a scope, and a stool, albeit with some contortions necessary at times! 

Three in the hide and the little one said...
Photo by Helen Boland.

The purpose of placing a hide on one of the port structures is to find out whether Terns which hatched in the port are returning to the port to breed, and if they are, whether they are nesting on the same structures from which they fledged. (For a view from the hide, click the video below).

On the first trial, we got our first colour-ringed Common Tern, ‘PAL’. PAL was ringed by the Dublin Bay Birds Project in 2015 as a pullus (unfledged chick), on the nesting structure in the Tolka Estuary, making this bird four years old. Common Terns become sexually mature from three to five years of age and seeing this bird landing within the nesting area of the CDL Dolphin, at least suggests that it has returned to the port to breed, potentially for the first time!

Common Tern ringed at Sandymount Strand in 2018.
Photo by Helen Boland.

Our second visit to the hide yielded yet another ring-resighting, this time of a Common Tern ringed by BirdWatch Ireland in the Port in 2011 on the SPA-designated nesting platform owned by ESB. This bird, at eight years old, is of definite breeding age!

Monitoring and working to protect a species and habitat always comes with its own unique set of challenges, and the Tern colony in Dublin Port is no exception. This year, one nesting platform in Dublin Port has unfortunately suffered heavy predation, but by what we aren’t yet certain. A motion-activated camera has been placed on the breeding structure to try to ascertain what exactly is going on, and once armed with this information we will hopefully have a clearer idea of how to protect the birds nesting within this sub-colony.

Terns in Dublin Port.
Photo by Helen Boland.

Last season, two structures were affected by rat predation. This year we have been monitoring for the presence of rats on all four structures, using non–toxic chocolate and coconut flavoured wax blocks. Rats have a sweet tooth, and the idea is that if they are indeed present that they would chew the flavoured wax blocks leaving us with evidence of their presence in the form of rat tooth marks. This is a very simple method which has proven effective and inexpensive. Some rat activity was recorded at the beginning of the season on two of the four structures, but thankfully thus far there has been no direct evidence of rat predation within the colony. These pressures highlight the need to continue to monitor and protect the Tern population breeding at this site. 

Putting the wax blocks in place.
Photo by Helen Boland.

Monitoring block which has been chewed by rats.
Photo by Helen Boland.

Although this season has had a few bumps in the road, we are hopeful of a successful season for at least three of the four structures, and we are thrilled to have seen the first Tern rings on the structures. Identifying the predators which are affecting the sub–colony on the fourth structure will help us to further understand the pressures which can impact the breeding success of this colony.

Terns overhead during nest censusing.
Photo by Helen Boland.

Mallory, M.L. 2016. Reactions of ground nesting marine birds to human disturbance in the Canadian Arctic. Arctic Science, 2: 67–77.

Stoddard, M.C., Kupán, K., Eyster, H.N., Rojas-Abreu, W., Cruz-Lopez, M., Serrano-Meneses, M.A., Kupper, C. 2016. Camouflage and clutch survival in plovers and terns. Scientific Reports, 6: 32059.