Dalkey Arctic and Common Tern colony mid - season update

Guest Blog by Tara Adcock, BirdWatch Ireland

It’s been an exciting few weeks on the Dalkey Islands, with the first Tern eggs discovered on the 22nd of May, followed by the first Tern chick of the season on the 13th of June

The first Arctic Tern chick of the 2019 season on the Dalkey Islands. Photo by Tara Adcock taken under NPWS licence.

​The colony is split between three islands; Maiden Rock which has the only Common Tern pair nesting on the islands this season, Lamb Island, and Dalkey Island, which is by far the largest of the three islands.
 
Thus far, we have 14 chicks (all Arctic Terns), and 16 nests with eggs. The chicks are now getting big enough that we have been able to carry out the first ringing visit to the colony.
 
Both Lamb Island and Maiden Rock are restricted from access to members of the public during the breeding season to protect ground nesting birds. We also ask that on Dalkey Island, visitors are mindful of breeding birds, keep all dogs on leads and do not get too close to the sub - colonies. A good rule of thumb is, if the birds are acting agitated, you're probably a little too close. :)  

The Dalkey Islands complex. Maiden Rock on the left, Dalkey Island on the right, and Lamb Island to the right of centre. Photo by Karen Varnham.

Arctic Tern chick. Photo by Tara Adcock. and taken under NPWS licence

This year is the first time over the course of the project that Tern chicks have survived long enough on Dalkey Island to be ringed. In fact, these are the first Tern chicks to ever be ringed on Dalkey Island itself!

The first ever Arctic Tern chick ringed on Dalkey Island. Photo by Tara Adcock under NPWS licence.

​This is an amazing result which is due in large part to the tenacity of their parents, which are quite literally not afraid to knock some heads in the line of duty!

Arctic Terns on Dalkey Island dive bombing Stephen Newton. Photo by Tara Adcock taken under NPWS licence.

Over the winter we carried out rat baiting on both Dalkey and Lamb Island. Rats are a serious concern when present in a seabird colony. Seabirds have evolved to nest on islands and cliff faces as these sites were traditionally free of mammalian predators. As a result, these ground nesting birds have not evolved adequate defences against potential predators such as rats (Rauzon 2007).
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 At the end of the baiting project, it appeared that rats had been successfully cleared from Lamb Island but had not been completely removed from Dalkey Island. However, the ability of these Tern chicks to survive thus far on Dalkey Island suggests that the rat population is significantly reduced. Fingers crossed that these little guys will fledge and survive to come back to the Dalkey Islands to breed in five years’ time!

Arctic Tern chick Photo by Tara Adcock taken under NPWS licence.

Chris Johnson of BirdWatch Ireland assisting with the 2018/19 rat baiting project on Dalkey Island. Photo by Tara Adcock.

​The rings which these Terns have been fitted with will feed into a wider understanding of how long these birds live, the rate of survival for this species, and their migration routes. Not bad for a tiny piece of bling! 

First colour ring of the season on the Dalkey Islands. Photo by Tara Adcock and taken under NPWS licence.

Colour ringing has revealed previously unknown facts about the lives of many species of birds, including Common and Arctic Terns. For instance, we know from recoveries of ringed Common Terns which bred in the UK and Ireland, that these birds overwinter in the western portion of Africa, as far south as Namibia (retrieved from BirdWatch Ireland’s Dublin Bay Birds Project).

Resightings of ringed Arctic Terns and at sea surveys also provided the first indications that these birds migrate to Antarctica where they overwinter before returning north, some as far as the Arctic Circle, to breed. Geolocators have proven these theories correct in recent years, and also revealed that these birds fly a circuitous route along the coastlines of South America and Africa, using prevailing winds to help them along.
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Arctic Terns, weighing less than 125 grams or a quarter of a bag of sugar, migrate the equivalent of three to nearly four round trips to the moon in their lifetime! (Barrett 2016; RSPB 2018).  All going well, the Common and Arctic Tern chicks on the Dalkey Islands will begin their journeys to Africa or Antarctica respectively, at the end of the season.

Arctic Tern chick. Photo by Tara Adcock and taken under NPWS licence.

A successful Tern breeding season on the Dalkey Islands is vital to the long – term success of the colony. The last couple of years have seen successive poor breeding seasons for these species on the Dalkey Islands due to predation, which has led to a reduction in the number of returning Common and Arctic Terns to the nesting grounds.
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Terns are colonial breeders, which means that they typically prefer to nest in groups. This provides better defence against potential predators, as the more birds mobbing an intruder, the less likely it is to snatch eggs or chicks, and it is also less likely that a pairs brood will be predated. Therefore, the smaller the colony, the more vulnerable it is to predation.

Common Terns breeding on Rockabill Island. Photo by Brian Burke and taken under NPWS licence.

​Both Common and Arctic Terns are Amber listed species according to the Birds of Conservation Concern in Ireland. This means that although they are not considered on the verge of extinction in Ireland, they are vulnerable. By contrast, the Roseate Tern, a cousin of the Common and Arctic Tern, is considered Europe’s most endangered seabird. The Roseate Tern is less feisty than its more aggressive cousins, and as such prefers to nest among large colonies of Common and Arctic Terns upon whom it depends to help drive away intruders. 

Roseate Tern on Rockabill Island. These birds, unlike Common and Arctic Terns, prefer to nest under shelters such as nest boxes. Photo by Brian Burke and taken under NPWS licence.

The Dalkey Tern Conservation Project is part of the larger EU LIFE Roseate Tern Recovery Project. One of the aims of this project is to reintroduce the Roseate Tern to the Dalkey Islands, where as recently as 2004 eleven pairs bred. The reason it is considered critical to re-establish a Roseate Tern colony on these islands is because one colony in Ireland, Rockabill Island off the coast of north Co. Dublin, supports over 50% of Europe’s breeding Roseate Tern population. ​

Rockabill Island. Photo by Brian Burke

The history of the Rockabill Roseate Tern colony is fascinating, with numbers growing from just 152 pairs in 1989 to over 1,600 pairs last year, thanks to intensive and ongoing management by BirdWatch Ireland. However, this high proportion of Europe’s breeding Roseate Tern population on one island is akin to having all of your eggs in one basket. A number of poor breeding seasons on Rockabill Island could have long term consequences for the European population of this species.

​Therefore, diversifying the availability of suitable breeding habitat is crucial to the long - term success of the Roseate Tern.

Roseate Tern on Rockabill Island. Photo by Brian Burke and taken under NPWS licence.

To do this, we need to either remove predators such as rats through baiting, or thwart potential predators such as gulls using a 1 metre by 1 metre grid pattern of canes in the Tern colonies. This reduces access to gulls to the nesting sites on the Dalkey islands. The three species of gulls present throughout the breeding season have wings spans of 1.25m and above, while the wing spans of the Terns are .75m and smaller. Therefore, the idea is that gulls passing overhead simply will not fit between the canes and therefore have less access to the Tern nesting site.
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To increase the number of Arctic and Common Terns nesting on the islands, we hope to use decoys (wooden or plastic figures which look like Terns), and audio of Common and Arctic Terns to attract these species to the islands next season. 

Liam Gaynor recording the Arctic Tern nest he’s just found. Grid pattern of canes to exclude passing gulls in background. Photo by Tara Adcock.

​Finally, we’ll be walking on eggshells (not literally!) over the coming weeks and keeping everything crossed for a successful year, but so far this season is looking good! However, we are not out of the woods yet as Tern chicks are extremely vulnerable to predation, weather events and disturbance from people and dogs. We’ve put out chick shelters, nest boxes, gravel and canes to give them the best chance possible and will continue to monitor and to the best of our ability protect the small but mighty Tern colony on the Dalkey Islands. 

Terns off the Rockabill island shoreline. Photo by Brian Burke and taken under NPWS licence.

If you want to find out more about the Dalkey Island Tern Conservation Project, feel free to drop by the Tern Watch Events on Tuesday nights from 5-8 pm at Coliemore Harbour this July. The Dalkey Tern Warden and members of the South Dublin BirdWatch Ireland branch will be on hand to show you the Tern colony, the surrounding wildlife and answer any questions you may have. Telescopes and binoculars are available to use!
 
In addition, morning Tern Watch Events will be held at Coliemore Harbour from 11 – 1pm on the on the 14th and 28th of July. Following these, guided walks will be held on Dalkey Island at 2:30 and 4:15 pm, meeting at the pier on the island. For more information see the BirdWatch Ireland Facebook page and website. Thanks to everyone who has come along, I've enjoyed meeting you all, and seeing returning faces at the events! 
 
And finally, a massive thanks to Ken, Johnny and David Cunningham who have ferried us back and forth free of charge during both this and last year's nesting season. This is an incredibly generous act, and one which is greatly appreciated! If you have a chance to take the boat across to Dalkey Island with Ken and his brothers, I'd highly recommend it! 
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Tern Watch Event. Photo by Tara Adcock.

Tern Watch Event. Photo by Tara Adcock.

Guided walks on Dalkey Island. Photo by Des Burke-Kennedy.

Barrett R. (2016). Upwind or downwind: the spring arrival of Arctic Terns at Troms, North Norway. Ringing and Migration, 35, 23-29.
 
Rauzon M.J. (2007). Island restoration: exploring the past, anticipating the future. Marine Ornithology, 35, 97–107.

RSPB (2018). Everything you need to know about Arctic Terns. Royal Society for the Protection of Birds, 1st May. Available at:
ttps://community.rspb.org.uk/ourwork/b/natureshomemagazine/posts/everything-you-need-to-know-about-arctic-terns. (Accessed 27th June 2019).