The flight of the navigators - birds migrate from the Cheshire skies
October sees our skies filled with a thousand journeys from the Arctic to Australia, but how do our feathered visitors do it? Cheshire Wildlife Trust's Tom Marshall finds out
For a list of Cheshire Wildlife Trust reserves log on to www.cheshirewildlifetrust.org.uk
Turn right in 200 yards. Go left at the next roundabout. Perform a U-turn when it is safe to do so. So far, despite driving around 30,000 miles a year I’ve managed to avoid the use of a Sat Nav during their apparently unstoppable mass-migration from the shelves of Halfords into our cars.
Generally I rely on an as yet, unexplained geographical photographic memory – albeit one that relies on public houses a little too often. That said, and even with the assistance of the almost obsolete coloured parchment know as a road atlas, I still get lost occasionally. So quite how a bird smaller than a TomTom and minus the technical support of a dozen NASA satellites can find its way from a barn in rural Cheshire to sub-Saharan Africa and back again every year seems nothing short of astonishing.
The spring and autumn migration takes place on a grand scale – and whether it’s a dunlin or golden plover making the short journey down from our high peaks, to an arctic tern preparing to fly half way around the globe or perhaps a young swift embarking on a lifetime in the air that might clock up over a million air miles, the UK really is an international hub.
The truth of it is, for the most part we still don’t really know how this miracle of migration from Mother Nature takes place. It could be an innate understanding of the stars that would make Dr Brian Cox look like an amateur, or perhaps a built-in compass set to the earth’s magnetic field that draws swallows, wading birds and even butterflies halfway across Europe and beyond in an unstoppable urge. Either way, this October there’ll be hundreds of thousands of birds passing above our heads in a global flyway that would make the hardiest of air-traffic controllers break out in a cold sweat.
The trigger, it seems, is the reduction in daylight hours as swallows, house martins, sand martins and countless warblers – some of our tiniest species – will decide the time is right to head south.
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For some, migration can be quite a social event as swallows and other members of the Hirundinidae family gather together in reedbeds and other suitable habitats that offer a nightly refuge. There is of course safety in numbers, but those same flocks will naturally attract the attention of a passing sparrowhawk, or perhaps a specialist bird of prey such as a hobby – itself heading away from an unpredictable British winter.
For many species, the safest option is flying by night. This gives them the rapidly decreasing daylight hours to feed and rest, and then the cooler, denser air after dark to make their journey. These conditions mean less risk of dehydration, and less turbulence caused by thermals rising from the ground that could easily throw birds off course. Those same thermals however are all part of the flight plan for gliding and soaring birds of prey, for whom an overnight sortie offers much less attraction.
Rather less predictable than the behaviour of a honeymoon couple in the on-board facilities is the appearance of whooper swans at high altitude. Some of our largest and heaviest birds at around 10 kilos, whoopers have been seen by pilots at almost 30,000ft – about the height of Mount Everest. For their relatively short journey from Iceland, the whoopers will go for a non-stop flight of between 800-1300km, although this is still one of the longest European migrations entirely over water.
A swallow may cover up to 200 miles a day, but has some advantage in being able to feed en route, usually by simply opening their mouths! For wading birds however, like knot, dunlin and godwits looking to make the trip non-stop and with minimal refuelling, there is a need to take on some serious extra baggage in additional fat reserves – perhaps as much as half their bodyweight.
Perhaps one of the most incredible elements of migration is the ability for birds to find their way back to a single building, and in some cases even the same nest. Although the wider factors of the stars, the earth’s magnetic field, and even the sun and moon play a part, it’s believed that once within the local area some species may even use landmarks and smells to make the final twists and turns to their chosen summer or winter home. Following rivers and other natural ‘corridors’ can bring additional benefits to birdwatchers, who may flock themselves to migration bottlenecks or nature reserves along traditional routes, in the hope of seeing something out of the ordinary.
The only reason we know these remarkable journeys take place is through bird ringing schemes run by groups such as the British Trust for Ornithology and in recent years through the use of GPS tracking.
For almost a century however, our understanding of where birds go on migration has relied on the discovery of species such as swallows in other parts of the world, especially those fitted with a tiny leg ring bearing the details of the British Natural History Museum.
Young birds are often ‘ringed’ in the nest, while thousands of other birds are carefully caught in delicate nets at migration ‘hotspots’ before being released soon after bearing a new ring. Today, almost a million birds are ‘ringed’ or tagged in some way each year, leading to some of the most staggering finds in migration, such as the Arctic tern that will think nothing of a 22,000 mile trip from the UK to Australia – and back.
So next time you head out, leave the Sat Nav at home and try a compass, the moon or perhaps even the stars to guide you on your way. Whichever method you choose, you may have to wait a little bit longer than usual to hear those comforting words – ‘you have reached your destination’.