How can migrating birds fly over 10,000 km without eating anything?
― Their fuel is fat, stored up to nearly half their body weight
Some birds fly for eight straight days over open ocean, with no rest, no food, and nowhere to sleep. The secret is turning nearly half their body weight into "fat" before they even take off. Fat is the lightest fuel any living thing can carry.
An autumn tidal flat. A bird with a long, upturned bill is busily plucking ragworms from the mud. This is the bar-tailed godwit, a "bird of passage" that only stops in Japan briefly each spring and autumn.
Some of these birds fly straight across the Pacific, from Alaska all the way to New Zealand. The distance is about 11,000 km, with almost no land to land on along the way.
A human would feel faint after skipping just one day of meals. So how can a bird weighing around 400g keep flapping for eight days without eating a thing?
There are two main reasons
Fat can store far more energy than sugar for the same weight. Before setting off, the bird eats non-stop until fat makes up nearly half its body weight.
The stomach and intestines, unused during flight, are shrunk beforehand. That saved weight means it can fly further on less fuel.
Both come down to weight. For a flying creature, every gram of weight is fuel burned. That's why what to carry, and how much, becomes a matter of life and death.
Why is fat the "ultimate fuel"?
The body has two main ways to store energy. One is "glycogen," a chain of linked sugar molecules; the other is fat.
Gram for gram, fat holds more than twice the energy of sugar. Water widens the gap even further. Glycogen is stored in the body together with several times its own weight in water. Fat, on the other hand, holds almost no water at all.
Once you factor in that water, carrying the same amount of energy as glycogen takes roughly eight times the weight of fat. Look at Figure 1. Eight days' worth of fuel stored as fat weighs just 200g. Try to store it as glycogen instead, and it would exceed four times the bird's own body weight — far too heavy to even take off.
Most of the energy we humans store is also fat. But human body fat is at most around 20-30% of body weight. The bar-tailed godwit, by contrast, spends the weeks before departure feasting on ragworms and shellfish on tidal flats, building fat up to nearly half its body weight.
Flying after "offloading" the stomach and intestines
The other trick is even bolder. Studies of birds just before departure have found that digestive organs — the gizzard, intestines, liver, and others — shrink beforehand.
Since the bird eats nothing over the ocean, its digestive tools go unused for all eight days. Carrying them at full size would be a waste. So it's thought the bird shrinks them beforehand, redirecting that freed-up weight into fat and into the chest muscles used for flapping.
The body keeps changing during the flight, too. As fat burns off, the bird gets lighter, so it needs less energy to cover the same distance. Along with fat, protein from muscle and organs is also thought to be gradually burned as fuel.
Calculations put the average power output during flight at about 11 watts — roughly the output of one small LED bulb, sustained for eight days of flapping (see the final collapsible section for details).
Burning fat produces water along with carbon dioxide. This is called metabolic water. For a bird that can't drink over the open ocean, fat is thought to serve as both fuel and a source of water inside the body.
Departures from Alaska often coincide with tailwinds from low-pressure systems crossing the North Pacific, reports suggest. To save fuel, the birds seem to be "reading" the weather map.
Summary
Migrating birds can fly long distances without eating because they load up on fat — the lightest fuel available — until it's nearly half their body weight, and they shrink unused organs to travel light. Carrying the same energy as sugar would push their weight past four times their body weight, making takeoff impossible.
The bird rebuilds its own body into a fuel tank before flying.
Fat is the lightest fuel, chosen for a sky that shows no mercy to excess weight.
For how migrating birds avoid getting lost, see "How do migrating birds find their way without getting lost?"; for how flying in formation saves fuel, see "Why do migrating birds fly in a V shape?". For how the energy in food is measured, see also the article on "Calories".
- Get the nutrition labels of two foods. Salad oil and cooked rice make a clear comparison.
- Compare the energy per 100g of each. Oil comes to about 900 kilocalories, while cooked rice is around 150 kilocalories.
- Rice contains a lot of water. Confirm that the difference between "sugar stored with water" and "fat stored without water" shows up directly in these numbers.
In autumn (roughly August to October), shorebirds like the bar-tailed godwit can be seen on tidal flats across Japan. With a pair of binoculars, you can watch them feeding non-stop before their journey south.
Want to know more? ― Terms, formulas, and textbook connectionsLabels show whether each part is junior-high-school level or university-level, all the way up
- JHSCovered in junior high school science
- HSCovered in high school biology or physics
- HS+Advanced high school content, or textbook sidebar material
- UnivNot covered in high school — university-level content (biochemistry, comparative physiology)
- ResearchNot yet settled even at university level — an active research question
JHSTerms: this phenomenon has names
- Migration: The seasonal movement between a breeding ground and a wintering ground.
- Glycogen: A store of sugar in an animal's body made of many linked glucose units, found in the liver and muscles.
- Metabolic water: Water produced when nutrients are burned in the body. Burning 1g of fat is thought to produce about 1g of water.
JHSHSCheck with a formula: what power output does the stored fuel give?
Let's estimate the stored energy and average power (rate of work) for a 400g bird with 200g of fat, flying 11,000 km in 8 days (192 hours).
| In symbols | E = m × e, P = E ÷ t |
| In words | Stored energy = weight of fat × energy per gram. Average power = stored energy ÷ flight time |
| Where it comes from | Conservation of energy: the chemical energy in fat is assumed to convert entirely into flapping work and heat |
| Symbol m: weight of fat (example) | 200 g |
| Symbol e: energy per gram of fat | about 39 kJ |
| Symbol t: flight time (8 days) | 192 hours |
| Energy per gram of glycogen with water | about 4.5 kJ (estimated) |
| Stored energy E | 200 × 39 = 7800 kJ |
| Per hour | 7800 ÷ 192 ≒ 40.6 kJ |
| Per second (40.6 kJ = 40600 J) | 40600 ÷ 3600 ≒ 11.3 W |
| Used per km | 7800 ÷ 11000 ≒ 0.71 kJ |
| Same amount stored as glycogen | 7800 ÷ 4.5 ≒ 1733 g |
| How many times body weight | 1733 ÷ 400 ≒ 4.3 times |
| Converted to food units | 7800 ÷ 4.184 ≒ 1864 kilocalories |
The average power comes to about 11 watts, roughly one small LED bulb. The stored fuel amounts to about 1900 kilocalories, close to what an adult human eats in a day. In effect, a 400g bird packs a human's daily meal into its body and flies for eight days on it. In reality, protein is burned too, so this is only a rough estimate.
HSHS+Why does fat hold so much energy?
HSIn respiration, organic matter combines with oxygen to form carbon dioxide and water, releasing energy that is used to make ATP. Fat molecules are made mostly of carbon and hydrogen and contain little oxygen. Because they are less "already oxidized," burning them releases more energy.
HS+Sugar molecules already contain a lot of oxygen, and because they mix so easily with water, they're stored in a hydrated state. Fat repels water, so it can be packed densely without carrying any. This double difference — "per gram" and "per gram including water" — adds up to roughly an eightfold gap.
UnivMuscles built to burn fat fast: beta-oxidation and fatty-acid carriers
Fatty acids are broken down in mitochondria through repeated cycles of beta-oxidation, entering the citric acid cycle as acetyl-CoA. Ordinary mammalian muscle is generally poor at using fat as fuel for intense exercise, because water-insoluble fatty acids can't be transported quickly from the blood into the muscle. In migratory birds, however, proteins that carry fatty acids in the blood, along with fatty acid-binding proteins inside the muscle, are known to increase sharply during the migratory season. In comparative physiology, this is considered one of the keys to long-distance flight.
📖 For the derivation of the formulas and further reading: Beta-oxidation (Wikipedia, Japanese) / Metabolic water (Wikipedia)
ResearchWhat's still not fully understood
- When do they sleep while flying? In some other seabirds, brief sleep in one half of the brain at a time has been reported during flight. It's still not well understood how shorebirds sleep over eight days of flight.
- How fast do the organs shrink and regrow? Research continues into how quickly digestive organs are restored after arrival, and how the bird eats in the meantime.
- Where is the limit? In 2022, a young bar-tailed godwit was reported to have flown over 13,000 km. Given body size and fuel capacity, the upper limit of how far these birds can fly is still unknown.
In other words, even this article reflects only "what's understood so far." Advances in small tracking devices keep rewriting migration records almost every year.
Connections to the curriculum (by level)
| Level | Subject / Unit | Where in this article |
|---|---|---|
| JHS | Science, Year 2, "Animal body structure and function (digestion and absorption)" | The difference between fat and sugar, digestive organs |
| HS | Biology "Respiration," Physics "Work and power" | How fat burns, the 11-watt calculation |
| HS+ | Advanced chemistry, "Fats and oils" | Water-repelling fat vs. water-holding sugar |
| Univ | Biochemistry, comparative physiology | Beta-oxidation, fatty-acid carriers |
| Research | Behavioral ecology, ornithology | Sleep during flight, upper limits of flight distance |
| ― | Everyday connections | Food energy labels, autumn birdwatching on tidal flats |
- Gill, R. E. et al. (2009) Extreme endurance flights by landbirds crossing the Pacific Ocean: ecological corridor rather than barrier? Proceedings of the Royal Society B
- Piersma, T. & Gill, R. E. (1998) Guts don't fly: small digestive organs in obese bar-tailed godwits. The Auk
- Jenni, L. & Jenni-Eiermann, S. (1998) Fuel supply and metabolic constraints in migrating birds. Journal of Avian Biology
- Wikipedia, "Bar-tailed godwit" (オオソリハシシギ, Japanese)
- Ministry of Education, Culture, Sports, Science and Technology, Council for Science and Technology, Resources Investigation Subcommittee, "Standard Tables of Food Composition in Japan" (日本食品標準成分表, energy conversion factors)
※This article is a general-audience science explainer. The figures given are approximations meant to illustrate the underlying mechanism. Body weight, fat mass, and flight time vary widely between individual birds and between years. When observing wild birds, please keep a safe distance and be mindful of the tides on tidal flats.