Nature 2023 December

What do the ruddock* and Manx shearwater have in common?

One of the delights of living in a rural location is seeing the different birds that visit our gardens, the surrounding woodland, farmland and commons, be they an unusual or rare sighting or familiar bird like our long-standing favourite, the robin. We see birds that seem to be with us year-round, though this may not always be the case. What we take to be all year-round residents may be seasonal visitors, despite being indistinguishable in look and behaviour from locals.

As the UK is on the western edge of Europe its winter climate is usually much milder than the continent due to the Gulf Stream. This attracts birds from across the North Sea and Channel in the autumn and winter. Some, like redwings and fieldfares, do not breed here but feed up on berries and invertebrates. In spring there is an influx of visiting and breeding chiffchaffs, blackbirds, thrushes, starlings, woodpigeons, goldcrests and yes, even friendly robins (aka ruddock)*, all recognised as the epitome of very British resident garden birds.

Bird migration is part of everyday conversation and countryside folk lore, foretelling the arrival of seasonal weather changes. But why does this remarkable movement happen? It appears there are two general reasons – one or both of which may apply to a particular bird population. First, to exchange areas of diminishing food availability for ones with higher certainty of food supply and second to find suitable nesting and breeding locations.

Long-distance migration involves a country-wide or trans-continental annual movement involving hundreds to thousands of miles. The two different locations are often called ‘breeding’ and ‘wintering’ grounds, but it may not always be as straightforward as this. A third periodic movement can be added. Irruptive migration occurs where birds unexpectedly appear, infrequently but in large numbers. Waxwings might appear in July in eastern England, perhaps only once or twice every twenty years, when there are severe food shortages in Scandinavia. If East Anglia cannot satisfy their voracious appetites they could visit the Chilterns, feasting on red berried garden bushes or out-of-town supermarket cotoneasters!

Historically, the reasons why birds suddenly appeared or disappeared was a mystery. From the time of Aristotle all the way up to the end of the 18th century, it was believed birds disappeared because they hibernated during inclement weather. Even the most diligent of natural historians, Gilbert White, concluded that swallows hibernated in sand banks, or buried themselves in mud. It was left to Thomas Bewick (of Bewick swan fame) to challenge the mythology in 1797, by piecing together the journals of sea captains who recorded seeing swans flying across the sea or congregating on islands in the Mediterranean. In 1822, a white stork was found alive in Germany with an arrow, made from an African hardwood tree, through its neck. James Audubon, the French-American ornithologist, started ‘ringing’, birds by attaching silver threads to the birds’ legs. Others marked birds with ink to test the theory that the same individuals returned to the precise location annually.

Despite these insights, most migrations remained a mystery throughout the 1800s. The first recorded organised bird-ringing scheme using metal rings was in England in 1909 by Harry Willoughby, founding member of the British Trust for Ornithology. Observation and ringing were pioneered by Dr William Eagle Clarke in the early 1900s, who first discovered that birds follow ‘trunk routes’ for long-distance migration. In 1930, RM Lockley established the idea of ‘bird observatories’ which allowed birdwatchers to work together across the British Isles and later worldwide. Bird migration research now uses RADAR, GPS mapping, satellite tracking, geotagging, acoustic mapping, though ringing still has a major role.

How did long-distance migration come about? The first bird migrations were alongside the earliest period of bird evolution, millions of years ago, caused by permanent geological and habitat changes. 60,000 years ago, glacial advance of ice sheets spreading southwards drove birds to disperse further away from their normal habitat and breeding grounds to seek food. Those birds that chanced on a better feeding ground thrived, and on returning successfully raised more surviving offspring than those who stayed put. Through natural selection, migratory patterns somehow became ‘genetically hard-wired’ into bird communities, or the whole species. As the northern ice sheets gradually retreated, 15,000 years ago, climates warmed and habitats improved. Birds took advantage, moving further northwards to breed, but migrated annually back to the same southern feeding sites. So, the distance they migrated gradually increased.

Not all birds follow the exact route of their fellow migrators, but individual birds will usually travel the same route they took previously, unless weather conditions intervene. This is amazing, but even more amazing is how most young birds make their first journey on their own and later return to where they were born.

There are several theories how birds combine their various senses to navigate, such as gathering compass information from the sun’s position, the stars or the earth’s magnetic field. Those familiar with the route might use landmarks like rivers in daytime. Geese have set routes and can climb to 19,500 feet to make use of thermals. Smaller birds fly at lower altitudes over a wider front to avoid adverse weather. Migration is both perilous and stressful. Food shortages, weather, exhaustion, obstructions, floodlit buildings and predators all take their toll.

Perhaps the most widely known examples in the UK for extreme migration are house and sand martins that travel from Southern Africa in April-May to breed in northern Europe, returning south in October. Swallows and nightingales travel from sub-Saharan Africa across the Sahara, Morocco, Spain, the Pyrenees and through France to breed. Of all these remarkable journeys the most astonishing are the migrations of the Manx shearwater and arctic tern. The shearwater swaps the feeding grounds in South America, travelling across the Atlantic Ocean to breed during our winter on the islands of Skokholm and Skomer in Pembrokeshire and Rum in the Scottish isles. Ten weeks after hatching, the young birds depart for South America. Terns spend our winter off the Antarctic ice-pack and almost circumnavigate the globe to reach their breeding grounds off Orkney and Shetland. An amazing round trip of up to 60,000 miles.

So, next time you see that curious robin follow you as you turn over the flower bed, or investigate the bird table, it could be a friendly visitor from the continent. Make it welcome!

Chris Brown BEM
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