Coming soon to a beach near you – the return of Arctic Terns and other summer visitors
As I’m starting to write this (on the 15th March) I suddenly realise that the Redwings that have been around most of the winter seem to have disappeared. These small thrushes arrive in October from Scandinavia and Iceland. Many pass through the Outer Hebrides on their way to places further south in the UK but some will stay overwinter. In spring they return to their northern breeding areas. At the same time, far away in the Weddell Sea, just off the coast of Antarctica, another migrant species is about to start its long journey back to the beaches of the Outer Hebrides.
Most Arctic Terns arrive on our coasts in early to mid-May, after a 16,000 mile flight from Antarctica. Research using satellite geo-locators fitted to birds from Iceland and Greenland show that they follow prevailing winds on a long s-shaped route north. In the autumn Arctic Terns go the opposite way – down the eastern Atlantic to West Africa and then down the west coast of southern Africa or the east coast of South America back to the Weddell Sea. They then range widely along the coast of Antarctica almost to Australia before returning north the following spring. In one year their annual migration can be up to 56,000 miles. Arctic Terns commonly live for over thirty years. In that lifespan they will fly over 1.5 million miles and probably see more daylight each year than any other living organism.
When they get back here they nest on first year fallow machair, shingle beaches, rocky headlands and offshore islands. Their success will depend on a bountiful supply of sandeels offshore and an absence of predators such as mink. Success varies from year to year. The mink may be well controlled now, but a lack of sandeels is often a major problem. Warming seas are thought to have driven the shoals of sandeels away to the north and in many Scottish colonies of Arctic Terns very few chicks have been raised in recent years.
Over much of the UK, falling food supplies are thought to be one of the causes of the rapid decline of another of our summer visitors. As a child Cuckoos were everywhere in the countryside. Their calls a sure sign that spring was here. Most people growing up in the countryside in SE England will not hear a Cuckoo now, their population has declined by 56% over the UK as a whole since 1970 but most of this has been in the SE of England. Cuckoos in Scotland actually increased by 33% since 1995 (BTO State of the UK’s Birds 2017).
The reason Cuckoos are doing well here could be that pesticide usage in the countryside is so much lower than in SE England so we still have lots of “woolly bears”, the hairy caterpillars of the Garden Tiger moth, a favourite food of the Cuckoo.
At the moment (13th April) most Cuckoos are still in west Africa. Each year the BTO runs a Cuckoo tracking project using tiny geo-locators similar to those used on Arctic Terns. The first Cuckoos won’t leave Africa until the beginning of April and it’s mid to late April before the first ones are seen in the UK – a similar sort of arrival date as the Arctic Terns’. By then Swallows will have been back in the UK nearly a month and are recorded from the Outer Hebrides from early April. In 2019 there were exceptional early records of Swallows across the UK including a report on the Curracag Facebook page of one seen on Lewis on the 17th February. The very warm southerly winds that bought Painted Lady butterflies to the Outer Hebrides in late-February also probably helped early Swallows on their way here. They probably won’t help the later migrants such as Arctic Terns and Cuckoos though, they are still hundreds of miles away. If the wind swings to the north in late March and April, as it often does, then the arrival of summer visitors is likely to be held back. Sadly the warm weather in February wasn’t really much of a guide as to what happened later in the spring. The old saying “one swallow doesn’t make a summer” really is true.