The ability to spend part of the year on the other side of the world is one of birds’ most fascinating behaviours, and holds a multitude of mysteries, some still unanswered.
Why do birds migrate?
In Portugal, every year we’re visited by birds like the Common Crane and the Norhern Lapwing, which take refuge here from the inclement weather and scarce food further north. On the other hand, in Spring and Summer birds like the famous housemartins come from the south to breed here. These two scenarios illustrate the debate among scientists: did migratory behaviour evolve as a way to escape the blight of winter at higher latitudes, or as a way to avoid “crowding” in the tropics and ensure better chances of finding space and resources to raise young? To solve this mystery, scientists at the University of Michigan traced the family tree of around 800 American bird spcies, analysing where successive generations of ancestors bred and wintered. For most of those species, results trace a route from north to south: birds which initially lived in North America year-round gradually moved south in the winter, establishing the patterns we know today.
But there are some bird migrations which scientists still can’t explain. Like the ancient murrelet, a bird from the puffin family, which flies from Canada to Japan and China every year – an almost 8000km trip over the planet’s largest ocean, to reach a destination where conditions seem to be very similar to those it left behind.
How do they know when to leave?
One of the most important factors is length of day: when birds are getting ready to flee winter, shortening days are a sign that it is time to leave. But that can’t be the only sign they react to, especially considering that when they return to their breeding grounds, their departure point is in the tropics, where day length hardly changes. When scientists used artificial lighting to keep birds under constant day length, several migratory species still knew when to leave. How? On one hand, clues that weather is about to change, such as changes in atmospheric pressure, seem to induce birds to migrate.
Another signal seems to be food availability, with some studies showing that birds leave later in years when food is scarcer in their wintering grounds, probably because they need more time to accumulate the energy they need for the voyage. It’s likely that all these factors help determine when it’s time to leave, with their relative importance varying depending on the species and the situation.
How do they know the way?
Many birds follow the same route year after year, over thousands of kilometres, without needing a map or GPS. The mystery of how they navigate isn’t yet completely solved, but we have some clues. We know that birds can use the position of the sund and stars, and that they can sense the Earth’s electromagnetic field, although scientists haven’t yet been able to pinpoint exactly how they do so. When they repeat the journey, they also use visual references to confirm they’re on the right route. And we also know that their sense of smell can play an important role, especially for seabirds.
How do they recognise their destination?
When a bird embarks on its first migratory flight, how does it know when to stop? The answer, as in so many aspects of migration, varies from species to species. For birds that migrate in flocks, it’s simple: just follow the adults that have made the trip before.
For many seabirds, on the other hand, migratory routes are less fixed: in the northern hemisphere, birds fly south in so-called dispersive movements. A bird which winters on the African coast one year could winter in Brazil the following year. Even seabird species with more fixed wintering grounds appear to essentially follow an instinct to search for better conditions, and over the years they start to recognise places with food and shelter.
On their trip back to their breeding grounds, birds recognise the area they hatched in.
Why don’t they take the shortest route?
Sometimes, the answer is obvious. It’s perfectly understandable, for instance, that birds which can’t land at sea fly along the coast, and that they travel to Gibraltar rather than attempt a direct crossing of the Mediterranean. But other cases seem more extreme.
Arctic terns fly from the arctic to Antarctica every year – a 40000km round-trip. One would expect them to take the shortest route, but no. When scientists tagged these birds with GPS transmitters, they discovered that some fly almost 100 000km (more than the equivalent of flying twice around the globe). The birds are likely forced to take less direct routes in order to find resting spots along the way, and the wind probably also plays a part in those detours.
Birds fly at speeds which are comparable to typical wind speeds, so migratory routes are often partly defined by favourable winds. In Sagres, in the autumn, southwest winds often bring small songbirds, which become unable to fly south until the wind changes.
When the time comes to fly back to their breeding grounds, many species take a different, more direct route. This is because the earliest arrivals get to choose the best nesting spots, and the later you arrive the less likely you are to still find a partner. During the autumn migration there’s no such hurry, so the choice of route is probably more influenced by the availability of food and refuge along the way.
What’s the impact of human activities?
One of the most generalised effects of human activities over the last century is climate change. It’s hardly surprising that climate change impacts a process as intimately connected to seasonal variations as migration. Birds which reproduce in northern Europe, where spring is arriving earlier and earlier, are migrating sooner – but it may not be enough. Take the European Pied Flycatcher. In recent years, these birds have left their wintering grounds in the south earlier. But, thanks to the warmer temperatures, the insects they rely on to feed their young are breeding even earlier, so when the flycatchers arrive, the insect boom is over. Like the flycatcher, several other migratory birds have had their breeding compromised by this mismatch between how birds, insects and plants respond to climate change. And to complicate matters further, for many species these changes are not happening equally throughout their migratory route: leaving wintering grounds early (aside from providing less time to accumulate the necessary calories for the journey) can mean catching more storms and bad weather along the way, for in places like southern Europe spring isn’t yet arriving as early as in the north.
Between milder weather and greater food availability thanks to human presence, birds like the White Stork may be abandoning migration entirely in some regions, including the Iberian Peninsula.
For those which still undertake these regular voyages, humans bring other risks. For small songbirds that fly across continents, and which fly mainly at night, navigating by the stars, city lights can be disastrous. Even if they can avoid crashing into buildings (specially skyscrapers), they’re forced to use up extra energy, calling and fluttering about, disoriented, and exhaustion leaves them vulnerable to other threats. In the USA, Audubon created the Lights Out programme, which, similarly to SPEA’s lights-out campaigns in Azores and Madeira to protect seabirds, encourages people, companies and public institutions to turn off their lights during the most critical periods of bird migration.
Another threat, which is unfortunately very significant in Portugal, is illegal bird trapping. Every year, thousands of birds are killed or caught in traps, to be sold as delicacies or pets. A threat that SPEA is fighting, battling for better laws and working with the authorities to improve the efficacy of the fight against these crimes, in projects such as LIFE Nature Guardians.
Is it worth it?
The large number and variety of bird species which fly between territories every year indicates that despite all the risks, migration pays off. In a study published in 2017, researchers at the Max Planck Institute for Ornithology, in Germany, showed that blackbirds which migrate south in winter are more likely to survive than those that stay in central Europe year-round.
Bird migration seems to be here to stay, to the delight of birdwatchers and researchers who will be able to continue to unravel the mysteries of this fascinating behaviour.