Scientifically Speaking | Birds fly great distances. We are now learning how

Bird migration is a vibrant area of study. Work published recently in some of the world’s most prestigious journals have shed light on this once-enigmatic phenomenon

It is autumn in the Northern Hemisphere. I live in a part of the world where the climate is temperate and so there is a crispness in the air. Look up in the sky and you may see birds migrating south for the winter.

Birds prepare for continuous flight for weeks. They start to eat immense amounts of food. They store more fat and gain weight (AFP)

There are few things as awe-inspiring as seeing a flock of migrating birds. During migration, a small bird such as the blackpoll warbler can fly continuously for 80 to 90 hours using less energy than contained in a bar of chocolate. In Seasons of Life, Russell Foster and Leon Kreitzman marvelled at the remarkable efficiency of this bird during flight- “if it had been burning petrol instead of reserves of body fat, it would get well over 300,000 kilometers to the litre”.

Bird migration is a vibrant area of study. Work published recently in some of the world’s most prestigious journals have shed light on this once-enigmatic phenomenon. And as the climate crisis disrupts ecosystems, patterns and distances birds fly are shifting.

Not all the 9,700 or so species of birds migrate. But a majority of those that live in higher latitudes do. Today we know that birds travel immense distances, sometimes greater than 10,000 km, when they undertake annual migrations. Nowadays, we can track the migration of birds with GPS-enabled devices. But this was not always the case.

The ancient Greeks thought that birds turned into fish when they disappeared in winter. In the 1700s, many Europeans thought that birds flew to the moon in the winter or hid in mud.

Those theories disappeared in 1822, when a stork was shot in Germany after flying all the way from Africa with a Bantu spear stuck in its neck. Today, this historically significant “Pfeilstorch” (German for “arrow stork”) is housed in the collection of the University of Rostock in Germany, and it provided the first evidence of long-distance migration of birds. Quite remarkably, it isn’t the only one; many other storks impaled with African arrows were spotted in Europe, underscoring the incredible compulsion of birds to migrate.

Birds migrate for several reasons, which include avoiding difficult seasonal conditions, finding the best resources for breeding, avoiding predators and diseases, and finding food. In the Northern Hemisphere, there is little vegetation in winter, so migratory birds travel south.

However, the return journey is also important. At higher latitudes, there is an abundance of food in spring and summer. There is a feast of grains and insects.

Birds prepare for continuous flight for weeks. They start to eat immense amounts of food. They store more fat and gain weight. Muscles needed for flying also become larger. Birds also shrink some other body parts including their guts, which are not needed for flight. Those that fly long distances during the day can use the movement of the sun and terrestrial landmarks to orient themselves. But in this aspect, nocturnal migratory birds are most remarkable since they can use the pattern of stars in the sky to map out where they are.

Many migratory birds orient themselves to the earth’s magnetic field. On June 23, a study that made the cover of Nature found that the European robin has a light-sensitive magnetic compass in its eyes. This biological molecule called cryptochrome 4 is a “living compass”. The authors think that magnetic-sensing properties of the compass originate in quantum mechanical phenomenon. They showed that robins, which are nocturnal migratory birds, have cryptochrome 4 that is better at detecting weak magnetic fields than the same molecule from non-migratory birds.

The current research raises the tantalising possibility that migratory birds can “see” magnetic fields as they travel long distances. If these results hold up, they will show that birds use weird physics for migration that we haven’t found in other animals.

How do birds know when to migrate? Migratory birds have both biological daily clocks and seasonal clocks that are calibrated by the appearance and length of light during the day. If kept in captivity, some birds become restless at the time of year they are programmed to embark on long journeys. Birds use these internal clocks to figure out when it is time to migrate.

Many migratory birds are solitary but flock together during migrations to avoid predators. Some fly in a V-formation for better aerodynamics and energy conservation. Small diurnal birds, including most songbirds, also fly at night during migrations. Night cruising makes perfect sense. At night, there are fewer predators, less risk of dehydration, and more favourable winds.

Research published on May 7 in Science shows that great reed warblers, which typically fly long distances at night, can raise their altitude to above 5000 metres during the day to avoid heat and predators. This adaptation allows them to cruise long distances non-stop over inhospitable regions such as oceans and deserts.

Bird migrations predate human urban centres, but humans have changed the natural environment. Tragically, close to a billion birds die each year due to collisions with buildings. Research published in Proceedings of the National Academy of Sciences on June 15 found that light pollution is a major culprit. Yet, there is a simple and energy-efficient way to save many of these birds. By keeping non-essential lights turned off in buildings at night and dimming windows (especially during spring and autumn migration), many deaths can be prevented. Already, some cities in the United States are implementing this measure.

The climate crisis is the greatest global challenge of our generation. It is disrupting habitats where bird fly and where they rest. It is also changing the availability of insects and seeds. Birds are arriving and departing at times that are mismatched to their honed life cycles. As the crisis accelerates, many majestic migratory birds will face extinction.

Anirban Mahapatra, a microbiologist by training, is the author of COVID-19: Separating Fact From Fiction

The views expressed are personal