Ireland hosts five species of tern, migratory seabirds which breed on our coasts and inland lakes in the summer months before migrating to warmer climes in the winter. The roseate tern breeds in huge numbers on Rockabill Island, where the colony is vitally important to the species’ European population as a whole. Climate change and invasive species threaten our terns, but work is ongoing to protect them and ensure their breeding success.

There are five species of tern which breed in Ireland, all in the genus Sterna. All of these birds are only visitors to our island, arriving in the summer to breed, before migrating to the southern hemisphere in the winter to avoid the coldest Irish weather! All of our tern species feed mainly on marine fish, and breed colonially on islands – they lay their eggs on the ground in gravel scrapes, which helps prevent eggs rolling or being washed away. The hatchlings hide in thick vegetation or other shelter until they are able to fly, being fed by their parents regularly.

• The common tern (Sterna hirundo) is, as its name suggests, our most commonly seen tern, arriving in March and staying through until October. It is commonly seen along our coasts, diving for fish, but can also be found over our large inland lakes, particularly in Galway and Mayo.
• The Arctic tern ( paradisaea) is the next most common tern in Ireland, arriving at around the same time as the common tern, but leaving slightly earlier in September. It is not found inland, only around the coast, where it dives for fish, crustaceans, and will also hunt insects. This tern is famous for its migrations – it has been known to travel pole to pole, to take advantage of both the Arctic and the Antarctic summers. This is an annual round trip of 35,000km (and you thought your commute was rough!)
• The sandwich tern ( sandvicensis) is our third tern species, and while it has been encountered along all of our coasts, it breeds only in a few select locations, mostly in the north and west of the country. However, one of the largest Irish colonies of breeding sandwich terns can be found near Rosslare in Co. Wexford, at Lady’s Island. Several hundred pairs of birds come here to breed each year.
• The little tern, funnily enough, is our smallest tern species, and is a rare visitor. It can be found along the coast mainly in the east and west of the country, arriving in April and leaving in late August. Most of the colonies are found in counties Wexford, Wicklow, and Louth – Birdwatch Ireland help to maintain a colony for these birds at Kilcoole, Co. Wicklow.

  

• Finally, the roseate tern ( dougallii) is our rarest and most localised tern species – it is found only in the east, with one colony on Rockabill Island (off Skerries), and another at Lady’s Island, Co. Wexford. While breeding has been seen at Dalkey Island in Dublin and the Blasket Islands in Co. Kerry, these colonies are relatively small. The Rockabill colony is the most important in all of Europe for roseate terns, hosting around 1,600 pairs of birds each year.

  

So how can we tell apart these four species?

• The common and Arctic terns are most likely to cause confusion, as both are fairly common, and look superficially very similar. The head and bill of the common tern is longer than that of the Arctic tern, but this might be difficult to tell unless you have two birds together!
  The common tern’s tail streamers (similar to those of a swallow) are fairly short, and do not extend past the tips of the wings when the bird is at rest – those of the Arctic tern are much longer and extend past the wing-tips.
  When in flight, the trailing edge of the common tern’s primaries (its biggest flight feathers, located near the tips of the wings) shows a diffuse black band, fading out into the white of the wing. The Arctic tern features a much sharper, less diffuse black band in the same location.
• The sandwich tern is our largest tern, and this is how it can be told apart from other terns – it is close in size to a black-headed gull. In addition, it has a yellow tip to its bill, and if you can see this, you know you definitely have a sandwich tern!
• The little tern is on the opposite side of the scales, our smallest tern species. It is easily distinguished from sandwich terns by this, and from other tern species in its summer breeding plumage by its yellow bill with a dark tip, as well as by overall body size.
• The roseate tern should be fairly easy to recognise – unless you are on Rockabill or Lady’s Island, its probably not a roseate tern! In the unlikely case that you need to compare a roseate tern to another species, it can be told from sandwich terns by its much smaller size and its bright red legs (the sandwich tern has black legs). The roseate tern has longer legs than either the Arctic or common terns, and importantly lacks the dark trailing edge on its primaries.

Our terns face a number of conservation challenges. They feed mainly on marine fish such as sand-eels, so any disturbance to the marine ecosystem could have a drastic impact on our terns. Climate change has the potential to upset the food webs of our seas, possibly leaving our terns with less suitable food, and damaging their chances at breeding success. As well as this, rising sea-levels threaten terns in particular, since they lay their eggs on the ground. Introduced species such as rats, mice, and even goats can have a significant negative impact on tern colonies – rats and mice will eat eggs and tern chicks, while goats can inadvertently trample eggs.

Birdwatch Ireland is doing excellent work in trying to help our terns – on Dalkey Island, volunteers have set up nestboxes to encourage the breeding of roseate terns, set up garden canes to shield the  vegetation tern chicks shelter in from gulls, and have  laid poisoned bait to try to eradicate rodents on the island. On Rockabill Island, Birdwatch Ireland wardens now take responsibility for management of the island’s seabirds, clearing vegetation to make room for tern nesting sites, and setting up nestboxes. Their hard work has helped the roseate tern colony here rise from its crashed level of 180 pairs to over 1600 pairs! As well as this, their work has attracted even more Arctic and common terns, as well as other seabirds such as kittiwakes and black guillemots.

Fun Facts about Terns:

• Common terns are probably not able to recognise their own eggs – some nests have been found to contain 8 – 9 eggs, some of which don’t belong to the nester and may even be roseate tern eggs!
• Common terns have been seen to incubate non-egg objects in their nests such as stones, bottle caps, and even snail shells! They may do this to stimulate incubation behaviour, especially if they don’t have a full clutch of eggs
• Arctic terns can live between 25 and 30 years, and their migration distance during this time is more than 3 MILLION kilometres – that’s 4 round trips to the Moon!
• Arctic terns are excellent parents, having been seen to attack even polar bears to defend their young!
• Common and arctic terns both nest on the ground, and mark out their nests with many different material such as grass, twigs, stones, and even bones!

Written by Tom Murphy

References

https://birdwatchireland.ie/birds/common-tern/

https://birdwatchireland.ie/birds/arctic-tern/

https://birdwatchireland.ie/birds/sandwich-tern/

https://birdwatchireland.ie/birds/little-tern/

https://birdwatchireland.ie/birds/roseate-tern/

https://www.rspb.org.uk/birds-and-wildlife/natures-home-magazine/birds-and-wildlife-articles/migration/migratory-bird-stories/arctic-tern-migration/

https://birdwatchireland.ie/our-work/species-habitat-conservation/marine/rockabill-tern-project/

https://www.dlrcoco.ie/sites/default/files/atoms/files/dalkeyternleaflet.pdf

https://www.birdlife.org/sites/default/files/attachments/key_threats_climate_change.pdf

https://www.researchgate.net/publication/333756516_Common_Terns_Sterna_hirundo_incubating_Common_Garden_Snail_shells_Helix_aspersa_on_Rockabill_Island

Tara Adcock, Birdwatch Ireland Warden on Dalkey Island – personal communication