Tuesday 30 April 2019

Terning our attention to summer

Winter is finally ending and with its passing so too comes the departure of most wading birds from our shores as they move to breeding grounds further northwest and northeast. However, as numbers of wading birds drop we barely have time to catch our breath before the Tern season is upon us. Even now, numbers are beginning to build as these tiny seabirds return to our shores eager to get on with the business of raising the next generation. So to celebrate both the turning of the seasons and the arrival of these beautiful sea swallows (as Arctic and Common Terns are affectionately known), we thought we would provide a brief overview of our work on the Tern colonies in Dublin Port.

Terns flying overhead as we land on one of the structures in Dublin Port Helen Boland

When picturing seabird colonies, an urban port is probably one of the last places we would expect to find a bustling seabird colony. Yet for Common and Arctic Terns, Dublin Port has long been an important breeding site, with records dating back to at least 1949 (Merne 2004). 

An All–Ireland Tern survey in 1984 found that 61 Common Tern and 30 Arctic Tern pairs were nesting at this site (Whilde 1985). Since then, numbers breeding in the port have increased significantly. Nearly 600 Common and Arctic Tern nests were recorded in Dublin Port in 2018 during censusing work carried out by BirdWatch Ireland’s Dublin Bay Birds Project. 

Tern chicks on the Great South Wall pontoon Helen Boland

So what makes the port ideal for these breeding Common and Arctic Terns? One factor is food, and Dublin Port is perfectly situated for this purpose, with plenty of small fish such as sprat and sandeels, the Terns preferred prey. A second consideration is safety of their young from predation. Seabirds have generally evolved to nest on islands and cliffs as these habitats provide a degree of safety from mammalian predators.

Common Tern offering a sand eel to its mate Neil Harmey

In Dublin Port, infrastructure such as pontoons and dolphins (large wooden structures with a concrete or metal platform rather than the loveable cetaceans) act as mini islands, ideal for nesting terns. Four structures (two dolphins and two pontoons) support colonies of Common and Arctic Terns in the port. The two pontoons were deployed by Dublin Port Company in 2013 and 2015, specifically for this purpose. One of the dolphins, owned by the Electricity Supply Board (ESB), is now part of the South Dublin Bay and River Tolka Special Protection Area due to its significance as a Tern breeding site. The continued commitment of the Dublin Port Company and ESB to providing a safe space for these breeding birds is one of the reasons why these colonies are thriving.

ESB Dolphin (SPA) Helen Boland

Not only do we carry out Tern nest censusing work in Dublin Port, but we also undertake significant ringing efforts of chicks at the site. Last year alone, over 650 Arctic and Common Tern chicks were fitted with a BTO metal ring each of which has a unique inscription used to individually identify birds. In addition, 125 Common Tern and 10 Arctic Tern chicks were fitted with colour rings (and hundreds more since 2015), each of which also has a unique code. The inscriptions on colour rings, unlike metal rings, are large enough for volunteers and surveyors to read in the field using telescopes (albeit with quite a lot of patience!).

Ringing Tern chicks Helen Boland

The aim of this work is to try to discover whether terns return to breed on the same structure on which they hatched, or if they return to the colony at all. To aid us with this important work, Dublin Port Company have installed a hide (a small hut used to unobtrusively spy on birds), on one of the structures this year. This hide is based on the design used on Rockabill Island (north Dublin) which is home each summer to ~80% of northwest Europe’s breeding Roseate Tern population, and where BirdWatch Ireland manage a conservation project each summer. The installation of this hide marks an exciting new chapter in the Dublin Bay Birds Project. The timing could not be more perfect as the colour-ringing began in 2015, and chicks ringed in that year could now have potentially reached breeding maturity and could be prospecting for nest sites on the Dublin Port structures. All we need now is a member of staff, a telescope, a bit of patience and a sprinkling of luck and hopefully we can begin to learn about the fidelity of terns in Dublin Port to their natal structures.

The new hide provided by Dublin Port Company on the CDL Dolphin Tara Adcock

Colour-ringing also tells us a great deal about the areas both nationally and internationally which these birds are using, and their migration routes. Terns are ringed by the DBBP not only at Dublin Port, but also at Sandymount Strand in early autumn. At the end of each breeding season, literally thousands of Terns which have bred in Ireland and further afield gather at Sandymount Strand in post breeding aggregations. See here for a blog on these phenomenal aggregations. Terns are caught at this site using mist nets (large nets stretched between two poles) and colour-ringed before being released. So far, our colour-ringing efforts have yielded 73 resightings of 30 birds ranging from Ireland, Scotland, England, Wales, the Netherlands, Norway, Gambia and Namibia!

Juvenile Common Tern chick with colour ring John Fox

Finally, the ability for nature to survive, not only on the edge of a bustling capital city but cheek to jowl with industry, is in no small part thanks to a great team effort from a combination of organisations. Critical to this, of course, is the support, both financial and otherwise, provided to DBBP by Dublin Port Company. In addition, access is facilitated by ESB, and licenses are provided by NPWS, but with great interest and advice provided by all. It goes a long way to show the significant and positive impacts big organisations can have on wildlife when they work together to provide space for nature.

Terns flush overhead as the boat approaches the colony Helen Boland

Merne, O.J. 2004. Common Sterna hirundo and Arctic Terns S. paradisaea breeding in Dublin Port, County Dublin, 1995-2003. Irish Birds. 7: 369-374.

Whilde A. 1985. The 1984 All Ireland Tern Survey. Irish Birds. 3: 1-32.


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