To migrate or not to migrate

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Some years ago, I watched a short film about captive-bred whooping cranes. Being so, they don’t know how and where to migrate. So humans taught them, by flying along the route in a micro-light aircraft, the eager bird students trailing behind. Just thinking of the sheer human audacity of teaching this (bird) life skill is a reminder of the how remarkable the instinct to migrate is.

Then there’s a bird that, in 2020, flew from Alaska to New Zealand. Non-stop. We know it did because of the satellite tracking device fitted on the bird. This is a distance of over 12,000 km, and this little bar-tailed godwit touched down 11 days after it took flight. So, it kept up a steady 45 kmph across a vast ocean with no land for hundreds of miles in any direction. The 12,000 km is a new world record, and it was set by a relatively nondescript, relatively small (about 300 gm) shore-bird.

All this might fill you with wonder, as it does me. How do you teach something that is normally pure instinct, and teach it across species boundaries? How does a small bird find the energy and strength for a 12,000 km flight?

But as interesting as those conundrums are, something else that migrating birds do is possibly worrying. Simply because they travel these great distances, they carry and spread micro-organisms across the world. Some of those can be dangerous to humans. Of course this isn’t new knowledge—we’ve known it for a while. But now, there’s a phenomenon that even birds might need to take into account: climate change.

Think of the weather, just in 2023. Early last month, for example, Phoenix broke a record of 53 days in a calendar year of temperatures over 110°F (about 43.5°C)—that record set only three years ago. Extreme rainfall has caused destructive flooding in Himachal Pradesh, Sydney, and now, New York. There have been wildfires and hurricanes around the world.

If all this is really evidence that temperatures are rising and moisture patterns are mutating around the world—that climate change is happening, in other words—then surely birds’ migratory habits will change as well. In fact, there’s evidence it’s already happening. Bird scientists are especially concerned about migration paths to or near the Arctic, because no other part of our planet is warming as quickly. In 2006, for example, a team of researchers reported: “Several bird species have advanced the timing of their spring migration in response to recent climate change…[In fact] long-distance migrants have advanced their spring arrival in Scandinavia…We argue that this may reflect a climate-driven evolutionary change in the timing of spring migration.” (Rapid Advance of Spring Arrival Dates in Long-Distance Migratory Birds, Niclas Jonzén et al., Science, 30 June 2006).

What these changes imply for the spread of pathogens is something scientists are trying to find out. For example, there’s a bird observatory and preserve in Sweden called Ottenby. For a lot of migratory birds, it is a temporary halt on their migratory routes. In the spring, they stop there on their way to spending the summer in points further north. In the autumn, they stop there on their way south, some flying on all the way to Africa.

During those stopovers, the birds do what birds (and maybe the rest of us) must: they excrete. Their droppings contain the pathogens they have been carrying. This contaminates the water bodies at Ottenby. In turn, other birds who stop in Ottenby may pick up those infections and may carry them to their final migratory destinations. Ottenby becomes, as a recent article remarks, “a global relay system for transmission” (Flight Risks, Science, 27 September 2023, https://t.ly/iBMIm). One Swedish ornithologist reports having found all these pathogens in birds in the 20 years he has been working at Ottenby: “influenza and Newcastle disease virus, Campylobacter, Salmonella, Yersinia, tick-borne encephalitis virus, Candidatus, Babesia, Rickettsia, and papillomavirus.”

But what if migratory habits change? Some scientists think that there are birds that, responding to rising temperatures, will fly faster while migrating in search of cooler places. That increased speed uses up more energy, and there are studies that show this makes them more susceptible to diseases.

Other birds might respond to climate change by choosing to stay put and breed in a spot like Ottenby, instead of using it as a stopover en route to somewhere else. This can, strangely enough, mean it will become a breeding ground for pathogens. If Ottenby is largely a stopover for many different birds, an infected bird is pretty likely to pass on its viruses to other bird species, in which the viruses may not survive. This is what evolutionary scientists know as the “dilution effect”: higher biodiversity decreases successful pathogen transmission. But if many birds of the same species make their home in Ottenby, that would negate the dilution effect.

There’s another, related, consideration as well. Birds in a migratory flock which are infected with a particularly nasty virus will tend to fall behind the others. That makes it more difficult to transfer the virus to other, uninfected birds. Thus, it may simply die out. But if the whole flock chooses to settle in Ottenby, the virus can easily make the leap to nearby uninfected birds. Thus, it may spread and survive.

Studying all this is no academic exercise. We know what happened with the 2.3.4.4b variant of avian flu virus H5N1, always called “highly pathogenic”. It originated in poultry in South Korea in 2014 and quickly made the leap to migratory birds. Birdwatchers “mapped in exquisite detail”—says the same Science article—how it travelled along migratory paths. By 2022, it had made its way through the Bering Strait to North America’s Pacific Coast, and through Iceland to the Atlantic Coast. This year, ornithologists have found the variant among birds in Greenland, which is warming quickly with climate change. The new places those birds choose to migrate to are, thus, potential new places for an outbreak of this destructive virus.

So yes, there’s something compelling about bird migration, also because it is a phenomenon so finely and delicately tuned to climate. The climate changes. What happens to that tuning?

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.

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Updated: 13 Oct 2023, 12:19 AM IST

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