How far can you run? One kilometre? Five? 10 perhaps? Maybe you're fit enough to run a half-marathon without too much fuss. Possible even a full one.
Whatever your record for distance might be, it's nothing compared to some of the planet's birds.
During a migration, certain species will cover hundreds of kilometres in one go - all without stopping for a snack or even a drink.
These birds’ incredible feats of athleticism have piqued scientists’ interest, because understanding the physiology behind them could give us a deeper insight into the fundamentals of biology and even improve our own health.
Globally, some 20 per cent of bird species are regular migrants, with the Arctic tern snatching the top spot for having the longest annual round trip, covering up to 90,000km (56,000 miles) as it travels almost from pole to pole.
Meanwhile, the bar-tailed godwit takes the prize for the longest distance travelled non-stop after one of them covered an astonishing 13,560km (8,425 miles) in a single flight – a giant step in a journey that enabled it to get from Alaska to New Zealand in 11 days.
Preparing for takeoff
Clocking up this sort of mileage, especially when travelling under your own steam, is an incredible feat of endurance. But why do the birds put themselves through it?
“We think the main reason birds have evolved to be migratory is so they can take advantage of resources, like food and safe places to breed, that are only available for certain parts of the year,” says Dr Guy Anderson, migratory birds programme manager at the UK’s Royal Society for the Protection of Birds.
“With more resources available, they’re escaping potential competition, so the birds that migrate produce more young and the habit then spreads throughout the population.”
It’s for this reason that migration is more common in the mid-latitudes, between the sub-tropical and polar zones, where temperatures and food supplies fluctuate throughout the year.
In tropical areas, nearer the equator, the climate and food supply tend to be more consistent.
The birds know when it’s time to migrate, thanks to environmental cues like day length, but there’ll also be hormonal signals that give birds a behavioural urge to start their journeys.
The changes in daylight cause the birds’ metabolisms to ramp up – their muscles grow larger, their aerobic capacity improves. And species that embark on longer flights will start eating like crazy.
“I was involved in a long-running shorebirds project in Delaware Bay on the eastern seaboard of the US,” Anderson says.
“Every spring, on their northern migration, red knots, ruddy turnstones, sanderlings, semipalmated sandpipers and others, would stop for two weeks to gorge on the eggs of horseshoe crabs.
"They’d double their weight and be like fat, little barrels – utter butterballs of birds – but all that food gives them the energy to fly up to the Arctic with either one or no stops.”
It’s this incredible ability that birds have to store and burn fat that makes them so different from mammals, such as humans.
Before birds set off on their migrations, some 50–60 per cent of their body mass can be fat, which they then use as a primary fuel source to power them through their flights.
“For endurance exercise in mammals – like us, for example – we can’t support intensive exercise with fat; it’s all carbohydrate,” says Dr Alexander Gerson, a professor at the University of Massachusetts, Amherst, in the US.
“If mammals get really, really fat, usually other things happen that aren’t great. Birds seem to avoid all that and can just take that fat and mobilise it really quickly.”
After completing their long migrations, most of that fat is gone – it’s been burned up. But according to Gerson’s research on migratory songbirds, a lot of the protein in their bodies gets broken down as well.
“Their guts are 50-per-cent smaller, their livers are 50-per-cent smaller, their muscles are 20-per-cent smaller. Even their kidneys are reduced, which generally is very bad. But their [bodies seem to be able to handle it],” he says.
Gerson and his team have been studying migratory songbirds to find out why so much protein is used up.
Using the Advanced Facility for Avian Research at the University of Western Ontario, Canada, which has a specialised wind tunnel built specifically for observing birds in flight, they’ve come up with a hypothesis: burning protein releases water in the birds’ bodies.
This is an important adaptation, because as they’re flapping away, the birds are exercising really hard and breathing heavily – just like when we’re out on a run or a bike ride.
Unlike us, however, the birds can’t stop for a drink.
According to Gerson’s research, it seems that during the first hour of flight, when there’s an immediate need for water or amino acids, around 30 per cent of their energy is coming from protein.
Once the birds get into a rhythm, less than three per cent of their energy is coming from protein – it’s almost entirely fat driven.
When they land, these birds can build all this protein back in a matter of days without suffering from any negative consequences.
“Our current research now is focusing on that recovery phase,” says Gerson.
“How are these birds able to rebuild muscles and organs so quickly? How does that affect function through the refuelling phase, and how might that be important for their migratory journey?”
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Energy cells
Prof Wendy Hood, from Auburn University’s Hood Lab in Alabama, in the US, is trying to get answers on how birds manage these epic migrations by studying them at a cellular level.
She and her team are looking at mitochondria, the little powerhouses within animal cells that take in oxygen and nutrients to generate adenosine triphosphate (ATP), the body’s energy-carrying molecule.
“We know a lot about the physiology of migration,” she says. “But it’s the mitochondria that are ultimately producing the ATP that the birds need to make these incredible flights.”
In order to study mitochondria, the team first needed to gather tissue. But they ran into a problem. After collecting the tissue, they only had about two hours to make their measurements before the mitochondria started to die.
If they packed up the tissue and took it back to the lab, it wasn’t viable by the time they began working on it.
A solution to this problem came in the form of an orange and navy motorhome called the ‘MitoMobile’. They drove this fully kitted-out ‘lab on wheels’ to bird migration sites, so they could gather live tissue in the field.
In the MitoMobile, the tissue could be broken into smaller and smaller pieces, then repeatedly put through a centrifuge at different speeds.
“Ultimately, we end up with a pellet of live mitochondria at the bottom of our test tube,” says Hood.
In 2024, the team’s first study using the MitoMobile was released in the journal Scientific Reports, where they looked at two subspecies of white-crowned sparrow, called the Gambel’s sparrow and the Nuttall’s sparrow.
The Gambel’s subspecies is a long-distance migrant that travels from California to Alaska and Canada in the spring, before making a return trip in autumn. In contrast, the Nuttall’s subspecies simply stays put on the California coast.
The team collected Gambel’s sparrows in April 2021 just as they prepared to fly north, then in September they caught the same subspecies as they were flying south. Nuttall’s sparrows were also collected in September.
In December, they sampled both subspecies while they were overwintering.
They found that throughout the year, the migratory birds had more mitochondria than the non-migrants. The difference wasn’t huge, however, and wouldn’t be solely driving the birds’ long-distance feats.
But digging deeper into the biology, the team found that mitochondrial performance in the Gambel’s sparrows was ramped up just before and during migration, allowing them to better process the oxygen and nutrients they needed to power their flights.
Essentially, migration causes them to switch from ‘cruise control’ to ‘sports mode’. In winter, the mitochondrial performance in the Gambel’s subspecies dropped off and became like that of the Nuttall’s sparrows.
But as well as switching up their performance, it also appears that the mitochondria can physically change shape.
“Mitochondria can go through these phases of fusion and fission. So, you can have mitochondria coming together and breaking apart,” says Hood.
While more work needs to be done, the theory is that when you have more fusion and more elongated mitochondria, you tend to have more oxidative phosphorylation – the process by which cells generate ATP.
And then fission can get rid of the parts of the mitochondria that aren’t working so well.
Hood and her team found that the migratory sparrows showed greater mitochondrial remodelling before and during migration, suggesting that this could play a key part in allowing the tiny birds to embark on these huge flights.
Hood is keen to stress that more work still needs to be done, however.
“These are foundational studies that help us start piecing together how mitochondria work overall,” she says.
“But maybe this study, with the results of other studies, could be really valuable in helping us go after certain mechanisms or drugs to benefit human health.”
This is a salient point, as an increasing number of human diseases seem to have a mitochondrial element.
Plus, poor mitochondrial function and the accumulation of defective mitochondria are closely linked to the ageing process.
If we could somehow slow down the changes that occur in our mitochondria as we get older, we could potentially stay healthy for longer.
Gerson agrees that unpicking the biology behind bird migration could help us to improve our health. Currently, he’s investigating the processes behind how birds can regenerate their muscles so quickly after their long-distance flights.
“Ultimately, there might be some sort of human health implication, where you’re talking about recovery from muscle-wasting diseases or things like that,” he says.
“Reduced muscle mass can affect mobility and all kinds of other things, and it’s a by-product of diseases like cancer and AIDS.”
Muscle-wasting in humans is currently hard to treat, so Gerson wonders if maybe birds could hold the key in terms of some metabolic pathway that’s increased during the recovery phase and could help in human health.
“It’s a very long-term goal, but it’s on our radar,” he says.
“Imagine a bird flew across the Gulf of Mexico and it’s totally depleted. Its muscles have been metabolised by 30 per cent.
"Its gut is 50-per-cent smaller than when it left Central America a few days earlier. What do they need to help them recover when they land? There’s also an ecological question here.”
And this ecological angle is a vital component of the bird migration story.
Vital pit stops
Migratory birds will navigate using a variety of techniques. They look for environmental landmarks like coastlines, rivers and mountain chains.
They even find their way using the Sun, stars and magnetic fields. Some species will simply follow other birds. Once an individual bird has successfully completed one migration, they'll follow the same route for the rest of their life.
While embarking on these impressive journeys, many species will need to stop for a few days to rest and refuel, so they rely on patches of rich habitat – perhaps a forest or a wetland – to land on at some point along their route.
Some of these sites have been used by birds for generations, as that’s what they’re familiar with. If these areas are suddenly destroyed or degraded, the birds won’t be able to take a break, so they’ll just have to keep going.
“If they find another one, then great. But remember, they’ve never been to the next one down the line, because they’ve never needed to,” says Anderson.
“And this is why having patches of good habitat is critical, because they rely on those ‘service stations’ to continue their migration.”
Aside from the degradation of their stop-off points, migratory birds are exposed to a greater number of potential hurdles than species that always stay in the same region.
On their journey, they may get caught out by bad weather, predators or problems associated with climate change.
Unregulated hunting may even be an issue at certain points of their trip.
As migratory birds travel such vast distances across so many countries, it’s a huge task for scientists to establish how best to study and conserve them.
What’s becoming clear is that we’re only just starting to uncover the remarkable abilities of these feathery athletes.
It’s going to be exciting to see how this scientific journey plays out and how conservation and our own health could benefit.
5 incredible bird migrations
Adélie penguin
13,000km round trip
Despite being flightless, Adélie penguins still manage an impressive migration.
The birds remain in the Antarctic, but migrate from their breeding colony to their overwintering ground by following the edge of the ice as it expands.
Bar-tailed godwit
11,000km each way
While they might not fly as far as the Arctic tern, bar-tailed godwits win the prize for the longest non-stop migration of any bird. In September or October, they fly all the way from the Arctic to overwinter in New Zealand.
Willow warbler
13,000km each way
This little featherweight tips the scales at a tiny 10g (less than a £2 coin or a 50-cent piece in the US), but still manages to traverse continents with few breaks.
In 2018, scientists tracked the plucky birds from their breeding site in northeast Russia to rest stops in the Mediterranean and southwest Asia, before heading to their overwintering sites in Kenya and Tanzania.
Sooty shearwater
64,000km round trip
Every year, these long-distance oceanic migrants nest in the southern hemisphere before travelling into the northern hemisphere and then back again, following a distinctive figure-of-eight route.
During migration, they can travel an impressive 1,000km in a single day.
Arctic tern
Up to 90,000km round trip
This species holds the record for the longest migration by distance.
Arctic terns breed in the Arctic and sub-Arctic during the summer months, then head all the way back to the Weddell Sea in Antarctica in September.
They don’t fly a direct route; instead, they’ll take advantage of prevailing winds to save energy.
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