Almost half of the world’s bird species migrate, and in this post, we’ll look at their most common routes, how wrong Aristotle was about why they leave, and answer some fascinating questions like what was the longest recorded bird trip.
One crisp September morning in downtown New York, 1963, Burt Bacharach and Hal David put the finishing touches to what would eventually become one of the world’s best loved songs, “They Long to Be (Close to You)”
Was it a coincidence that it was written at the same time as thousands of blackpoll warblers were making part of their annual 20,000km round trip, from Manitoba, western Canada, across northern US and down the eastern seaboard, right through New York State to rest awhile in the Carolinas, before a non-stop 4-day long flight to the Amazons? Did a sizeable flock of stripy, black-capped and moustachioed chubby songbirds just happen to alight in the thinning trees outside their apartment, just as the song writing duo laboured to find a suitable Disneyesque phrase?
Migration has been marvelled at throughout history; it is thought that early Pacific islanders, 3000 years ago, observed the flight patterns of golden plovers for their land finding techniques.
“Aristotle suggested the winter disappearance of birds was down to hibernation … it wasn’t until the late 18th century that migration was the accepted scholarly reason”
Aristotle carried out the world’s first study of birds in the 4th century BC. However, he posited that the winter disappearance of birds was down to hibernation, as well as, the somewhat less believed, theory of bird to bird transmutation, and whilst he did ponder the possibility of movement away, it wasn’t until much later at the end of the 18th century that migration was the accepted scholarly reason for the seasonal vanishing of the birds.
How far will you go, little bird?
As summer ends, swallows gather on the telephone lines, preening, chattering, possibly swapping route plans. In a few days they will all be gone, and I’ll no longer have to slow down whilst driving the dusty maize-edged roads through the countryside. Many a time I’ve sworn blind that I can’t possibly have missed those arcing torpedoes as they swooped and clamoured in the heat, consuming hapless cabbage whites, meandering mayflies, woebegone wasps, in their thousands. A glance in the rear-view mirror, and I can release my held breath in relief, as behind me, they just continue to feed, feed, feed, building up muscle and tendon mass to begin their fabled 320km a day, 7 days a week for six-week flight, south, to Africa.
When people talk about migration, most will think of the long-distance journeys, such as the ones our swallow friends will make. With the help of remarkable advances in technology, smaller and lighter transmitters have been fitted to many species of birds to learn more about distances travelled, and in 2010, the longest ever long-distance migration was recorded, made by the arctic tern. This tiny 4-ounce bird has been discovered to travel 71,000km in a single migration, racking up a mind-numbing 2.4million km throughout its life.
Long-distance migration is triggered, primarily, by notable increases or decreases of day length – the changing of the seasons. This in turn affects the abundance of food, and the ability to keep warm or to cool down. Birds who inhabit tropical zones tend to stay put – there is always food, and they have adapted over time to the heat and humidity of the land. But many species who inhabit temperate or arctic zones will need to move, and move fast, if they are to survive and further the family line.
As well as north-south migrations, there are many birds who also travel east-west; the blackbirds you see one day in your UK garden in March may well have just arrived from Estonia.
“The blackbirds you see one day in your UK garden in March may well have just arrived from Estonia.”
Not all of your friendly allotment-loving red-breasted robins have been there all year, either; were you to speak “Avese” (not a real language), you might detect a tinge of melodic Norwegian, or Romance from France, in their new territorial song.
Just popping to the shops
But there are other species of birds who do not make these famously arduous continent crossings. Some birds undergo short-distance migration, whilst others will undergo altitudinal migration, and it is thought that this relatively less rigorous type of travel is triggered by local weather conditions affecting food availability, rather than genetic programming believed by many to be responsible for long-distance migration. Waxwings, once having stripped the berries from the forests of northern Scandinavia, will wander southwards to Europe in search of more. On occasions, food will become so scarce if the summers have been particularly wet and grey, and they will fly across the seas to the UK, in a movement known as an irruption.
Chaffinches migrate too, albeit within a range of about 5km, but the chaffinch you fed earlier that week may not be the same one, and now your friend in another town down the road is making sure there are seeds to be mashed.
Then, there are those birds who don’t necessarily follow compass points, but go up, or down. Altitudinal migration, or vertical migration, is simply just that – birds that move from one contour of a map to the other, like the tragopans in the Bhutan Himalayas, moving from the snowy tree-less peaks to the lower shrub carpeted foothills, or the American dippers in the swirling rivers of Washington state, seeking out the dragonfly nymphs and crayfish trying to hide among the pebbles of the warmer waters downstream.
Finally, there are migratory birds who do so to moult. All birds moult, shedding those feathers that have become eroded and less efficient over time, or for display reasons during breeding season, changing their chest colours from pallid greys and muted browns to orange-edged russets and vibrant blues and yellows.
Moult migration occurs mostly in waders and ducks, and only then will sections of that society migrate – geese who have failed to breed that season, for example, whilst the successful families stay behind to rear their young.
But there is one species of duck, the Shelduck, that must migrate to moult. They lose all of their flight feathers at once, and this renders them mortally vulnerable. Therefore, just before this process begins, they flock in their thousands to the straits of Germany’s Wadden Sea, where, on and around the islands of Heligoland, they bide their time far from predators, whilst their new feathers grow.
Migration of all types, for all birds, comes at huge cost. Mortality rates are high along the way, not just from a simple lack of energy, or weather events, but also from predators, which includes, since time began, human hunting. Oceanic storms and predation of birds at rest will greatly affect the numbers of a species on the move, but other aspects of our own human interaction also influence the outcome of a bird doing its level best to stay alive. Now that the modern world is aware of what type of migration exists, the race is on to find out the how of migration, and what we do to influence those mechanisms, negatively or positively. Studies have been carried out all over the world to work out just what factors come into play, and we’ll be discussing those theories and the evidence that currently exists to support them in another post.
We’ll also talk more about our impact on the world around us, specifically habitat destruction. It’s important for us to understand the consequences, as it is the belief of many that the future of our species is intrinsically linked to the ongoing plentiful existence of birds, to the benefit of all. Suffice to say, we can no longer claim immunity from our actions, but there is much we can do to aid and rectify those actions. All is not lost!
We hope you’ve enjoyed reading about the different types of migration, and maybe next time you see a flock of geese go honking overhead, or you happen to hear a distant curlew call on a moonlit night, you’ll wish the voyager well, safe travels, and may you find home in the days to come.