How do migratory birds find their way ?
Geese flying
© Christian Moullec
Biologists on board of the "Alcyon", the boat that undertook an expedition "from the Mediterranean Sea to the Spitzberg", take turns to identify and count all the birds they meet. Among them, many are migratory birds.
It is springtime, that's why! Migratory birds are coming back to cooler areas to breed and build their nest. These birds often make incredible journeys and come back, year after year, to the same place.
Cock-and-bull stories and amazing facts
More than 2000 years ago, Aristotle (384-322 B.C.) had already discovered that some birds, like the cranes, were migrating in autumn. On the other hand, for birds like the swallows, he thought that they fell in a lethargic state and hid themselves in cavities or holes during the whole winter. In a word, he thought that swallows, storks, blackbirds and larks hibernated!
Many other crazy hypotheses have been made on the migration of birds: in 1703, a book suggested that, as it was impossible for birds to cross oceans, they spent the winter on the moon! Another well-known scientist, Carl von Linné (1907-1977), thought that swallows were spending the winter in the mud at the bottom of ponds...
Today we know that birds fly in a regular, recurrent and seasonal movement from their wintering ranges to breeding areas: nearly 40 % of the bird species living in Europe and Asia are migratory birds, which means that, in autumn, 3 to 4 thousand million birds are leaving Eurasia for warmer places, while other birds arrive from cooler countries. We now have a lot of information about these birds and their habits. Even though, migratory birds didn't loose their magic: the more we learn about them through scientific studies and the more we are astonished by their incredible feats and their fantastic capacities.
Each bird specie has its own route, with its own departure and arrival dates, its own way to travel (making many stops or flying as far and quickly as possible with nearly no stops) and its own habits (flying alone or with a group).
Bar-headed Geese fly over the Himalaya twice a year and go as high as 9000 meter to get over this obstacle!
The Arctic Tern, a bird weighing about 100 grams, makes each year the round trip between the Arctic and the Antarctic (about 20 000 kilometres) in order to never experience wintertime!
Now what about the Sedge Warbler, which can fly 4000 kilometres without any stopover? This bird weighs 23 grams when he begins the journey and only 9 grams when he arrives!
Other small migratory birds can fly during 75 hours straight at 1000 meters high to cross the Mediterranean Sea or the Sahara!
One of them made a short stopover on "Alcyon", while it was crossing the Mediterranean Sea:
22nd of March 2005
(...) At the back of the boat, a small winged dot on the sea is approaching Alcyon. It fights against the wind, it goes back a few meters, and then catches up with the boat and lands on the guardrail, catching its breath, just next to the tiller. No doubt: it is a Northern Chiffchaff, migrating, going back north to central Europe to breed. Quite adventurous, it hops around the deck, perches on the shrouds and finally inspects the cabin! After a few minutes, it leaves again, heading North. A Northern Chiffchaff. There is one nesting in my garden. I recognise his metallic "chiff-chaff" song. Next time that I will see it sing, perched at the top a bush, I will think of the open sea, of the storms that he had to endure and of this long journey. And also of the Berber that he probably crossed in the Moroccan oasis, and of the fishermens' boats that he landed onto for short stopovers (...).
Migration: a challenge!
Many would love to do just like migratory birds: live an eternal summer. But migration is not much fun: it is a real challenge.
Birds migrate mainly to search for food. But how do they know it's time to go?
For sure, it's not the lack of food that pushes them to leave, because they leave long before the food comes to miss. In fact, it seems that it's the length of the daytime and the temperature that influence the birds' hormones. That's how, in springtime, the birds "feel" it's time to leave. They then get ready for the migration: they stock "fuel" in order to have enough energy through the whole journey. In fact, during two weeks, they are going to eat a lot more than usual and, thanks to the hormone changes, their body is going to stock these reserves very quickly, gaining mainly grease. For example, the small Sedge Warbler, which we mentioned earlier, weighs normally 10 to 12 grams. Just before leaving for its migratory journey, it weighs twice more!
Many dangers threaten the birds during the journey: storms, predators, disappearance of their usual stopover places where they are used to find rest and food, etc. Only 20 to 30% of the young passerines survive to their first migratory journey. Among the adults, only half of them will come back the following year.
How do they find their way?
This question still interests many scientists. It has been showed that birds use several orientation "tools".
They can use the sun, for example, which means that they permanently "know" what time it is, in order to know the right direction on the basis of the sun's position. They are also sensible to the ultraviolet rays which penetrate the clouds but are invisible for human beings. Even the nocturnal birds use the position of the sun at sunset to know their position.
Nocturnal birds also use the stars. This has been proved by letting birds fly in a planetarium and changing the stars' position.
Another tool is the earth's magnetic field (earth's north and south magnetic poles). Some birds, like pigeons, have a small zone in their brain made of magnetite (magnetic mineral), just like a small compass. But other scientists think it's rather in their eyes that some birds have a system which indicates them where the magnetic north is...
Of course, birds also use their knowledge of the landscape: they follow rivers, valleys or roads, or locate themselves with particular mountain peaks.
Other tracks are still to be explored. For example it seems that some birds could find their way by following their sense of smell...
An experiment to save the Lesser White-fronted Geese: teaching birds a new migration route
The birds making the migration journey on their own, know their way by " instinct ". Others, flying in groups, have to learn the way with their parents during the first journey. That is the case of geese, cranes and swans.
This fact, along with a few others which seemed to have no link whatsoever, allowed Christian Moullec, a French meteorologist, ornithologist and pilot to launch an incredible project: teaching geese a new migration route.
- The first fact is that Konrad Lorenz discovered, around 1930, that if someone was present during the incubation of geese's eggs, as well as when the eggs hatch, the fledglings will take him as their "adoptive father" and will follow him everywhere.
- The second fact is that, in the eighties, Bill Lischmann succeeded to fly amongst migrating geese with his ULM.
- And finally, the fact that Lesser White-fronted Geese are an endangered specie, mainly due to the poachers in their wintering range, around the Black Sea and the Caspian Sea. In summer, these birds used to be seen all through Scandinavia. Nowadays they are only seen in Lapland.
In 1999, Christian Moullec "raises" thirty eggs of Lesser White-fronted Geese, hoping to teach them to live in new places, similar to their usual environments: the aim was to set their breeding area in a Swedish Wildlife Sanctuary and their wintering ranges in a German Wildlife Sanctury, away from the poachers. For the journey, Christian Moullec was forced to use a two-places ULM because the geese would not follow it if his wife, who had been more present when they were learning to fly, wasn't on board!
In autumn, the couple and the geese flew 1800 kilometres (900 above the Baltic Sea) to the German Wildlife Sanctuary in the Rhine valley. The experiment succeeded: in springtime the birds came back on their own to the Swedish Wildlife Sanctuary.
Christian Moullec just launched the second part of the project: with the help of german pilots, he is going to begin the same experiment once again, but this time with a lot more geese, to try to save this endangered specie.
