One good tern deserves another
The stamina and tenacity of the Arctic tern make it a fascinating visitor to our shores, says Keith Ringland
Walking onto a sandy Scottish seashore in late May or June I always listen for the sound of one of my favourite summer heralders. The Arctic tern (Sterna paradisaea) flying along the edge of the sea, calling out its sharp ‘kreeaah’ call, is synonymous with summer shoreline walks. These wonderful, noisy, graceful birds have one of the longest migration habits in the bird world so it’s always lovely to see them back from the South Atlantic.
Predominantly northern in their breeding distribution, up to this year Scotland held about three quarters of the United Kingdom’s Arctic tern population. The majority of these are in to be found in colonies in Shetland and Orkney, as well as the Hebrides and Argyll. But with avian flu running rife through bird colonies this year, from Shetland right down into England, we will all have to hold our breath to see how the birds emerge from this nasty epidemic.
Arctic terns are more abundant than the common tern, which has a more southerly distribution. They both look very similar but there are subtle differences. Arctic terns have completely blood red bills, whereas common terns usually have a more orange bill with a black tip. Arctic tern calls are deeper in pitch, slower and harsher than common terns, but it takes experience to reliably identify the species, especially at a distance. Pre-epidemic, most colonies were large to provide protection against predators. These slim birds are feisty to say the least when it comes to protecting their young. Venture too close, even innocuously, and the parents will very soon be flying over your head, screaming ‘kri-err’ repeatedly at you. Ignore this warning and you will find them diving direct at your head. Their pointed sharp bills can draw blood and they may also follow up by defecating on you.
However, these little birds need all the tricks in their arsenal as there is a wide range of predators – ranging from skuas, gulls, ravens, crows to even mere jackdaws – that have all been known to take tern eggs and chicks. Then there are the mammals: mink, otters, foxes and stoats, while on mainland Scotland, they can fall prey to hedgehogs, rats and cats. It is little wonder they prefer remote islands to nest, where they can at least avoid most of the mammals.
Arctic terns have consistently had the lowest breeding success rate of all our seabirds at 0.31 chicks fledged per pair between 1986 and 2019. Throw in major fluctuations in the sand eel population, the main food for feeding chicks, and there is little surprise that there have been dramatic falls and subsequent recoveries in Arctic tern numbers. Today they are rated as an amber-listed species, which means they are of moderate conservation concern.
Taking all these factors into consideration, I was very pleasantly surprised to find a small colony had established itself on a brownfield site within the town of Thurso on a recent visit to Caithness. There were about twenty-five pairs of Arctic terns together with a few common gulls rearing their chicks and no sign of any sick birds.
They had chosen their base on the stones of a former industrial site which had rosebay willowherb, docks, clover and a wide range of wild flowers now well established, growing between the stones. This mix presented open areas but also lots of cover of various heights in which the chicks could hide and shelter from the sun. The camouflage of the chicks was impressive and when they sat low and
‘They have one of the longest migrations of any bird in the world’
motionless, they were very hard to spot. So, what are the facts behind this plucky little bird?
Known as sea swallows because of their forked tail, buoyant, highly manoeuvrable flight and long migrations, they are a favourite of many nature watchers. They start to migrate to the southern hemisphere before our autumn, flying down as far as Antarctica, returning to the UK in late spring. This means they have one of the longest migrations of any bird in the world, travelling over 25,000 miles per year.
They feed mainly on crustaceans, insects and small fish, with the sand eel being a particularly important food source for successful breeding. Each one of these small silver fish is packed with oil, protein and energy. Exactly what young, growing chicks need if they are to survive.
Arctic terns almost rival gannets in their diving ability. Typically, a hunting bird would fly just offshore above shallow water until they spot the silver flash of sand eels below and then dive like a guided missile into the water and onto their target. The sand eels don’t know what hits them before they are being carried away by the bird.
Like many seabirds, Arctic terns hold captured eels crosswise within its blood red, ragged-edged bill. It is known that puffins have a tongue which they use to hold fish firmly against its bill, allowing them to go on to catch additional sand eels. In one of my photographs, a yawning Arctic tern showed its tongue and I suspect it may well use this to hold sand eels whilst going on to hunt more fish, similar to puffins.
As I watched the terns go about their business, it was obvious that one of the parent birds stayed to guard the chick or chicks, whilst the other one flew to the sea which was about a mile away to catch sand eels. This bird would feed the chick but also supply its partner with sand eels for sustenance as well.
Whilst guarding, the adult would rise to chase any other bird species that dared to pass close by, teaming up with the other adults to do so. During the quieter times, it would simply sit still, groom itself, readjust its feathers and even have very short light naps. The activity was always somewhat frantic with other guardian birds warning of any perceived threats.
It was a wonderful privilege to watch these birds go about their business, but as avian flu is now widespread and even affecting the only remaining roseate tern colony in Britain on Coquet island, off Northumberland, I have to say I am very concerned about the level of devastation that will sweep through these tern colonies. Will their numbers be dramatically reduced? It’s quite possible that the smaller colonies, having a reduced risk of infection, may go on to have a better fledgling rate than the large colonies. Only time will tell.