Self-Consuming, Literally!

Blog Author: Professor Adrian Thomas, CSO, Animal Dynamics

The skies over Oxford are full of swifts, screaming parties where the females race to chose the fastest (fittest) mates, the swallows are nesting under Donnington bridge, and there are Terns diving into the Thames after fish. All of these birds have migrated North to follow the sun to breed in the long productive days of summer. Adaptation for migration has pushed birds to the very limits of physiological performance, but there are birds that migrate from pole to pole (Arctic Terns, Godwits, Ruff) – the world is not large enough to limit them.

The flight performance of migrating birds is really amazing. Great Reed Warblers overwinter in sub-Saharan Africa, and breed in Northern Europe. Last week Sissel Sjoberg and her colleagues from Lund University, Sweden, reported in Science, that records from 1g onboard dataloggers showed the Great Reed Warblers crossed the Mediterranean in continuous flights lasting more than a day (longest 34 hours), and some climbed to over 6000m during that flight. Great Reed Warblers weigh 30g, that is the same as an Apple Watch, or 4 pound coins remember those? Tiny. No unmanned air vehicle could even come close, and they don’t have to carry guts and gonads. It turns out neither do migrants: guts don’t fly.

Decades of detailed research led by Theunis Piersma in the field, and by Thomas Alerstam and Anders Hedenstrom on radar, in the windtunnel, and in theoretical analyses has tracked the extraordinary physiological adaptations of migrant birds.

The record holders, for long-distance migration, are Bar Tailed Godwits, which fly non-stop from Alaska to New Zealand across the Pacific. They are fast fliers, but still take almost 200 hours for the flight (8 days). An extraordinary performance that requires a highly efficient flight-design, and Godwits have a beautifully streamlined body, long-thin-pointed high-aspect-ratio wings, and the ability to store huge amounts of fat as fuel. A 285g Bar Tailed Godwit can store almost the same weight in fat, and they need to – they burn almost 0.5% of body mass per hour during flight.

Scaled up to human size Godwits shed the equivalent of about 10kg of body fat per day, burning it to fuel exercise. A kg of body fat is about 7000 calories. Tour de France cyclists only burn 5000-8000 calories on a racing stage, and marathon runners only burn 2000-3000 calories. Migrant birds are working 10x as hard as the elite human endurance athletes.

Weight really matters for athletes, and more-so for migrants. The physiological adaptations are extraordinary: migrants fly between fantastically rich feeding sites (that is why they migrate) and at those stopover sites they fuel up fast, with their guts and gizzards working at peak rates. Serial measurements of migrant morphology at stopover sites has shown that they reduce their flight muscles and focus all their energy on feeding apparatus, to build up fat prior to the migration, but then in the last few days before the migration itself their guts, gizzards and livers (and other digestive organs) all shrink away, just as their flight muscles grow. Guts don’t fly. Measurements of migrants making long endurance flights in a windtunnel show that during the flight they burn fat at rates of 0.5% (Waders) to 2% (Hummingbirds) of bodymass per hour. But that means they get lighter, and as they get towards the end of the flight the motor (flight muscles) is larger than is needed to maintain flight-speed in their lighter (slimmer) form. So they burn the motor – using flight-muscle as fuel towards the end of the flight. As the fat-load and flight muscle get lighter the heart has less work to do to keep blood flowing, and towards the end of the flight some migrants even burn their hearts as fuel. What does this mean? It means that migrants can exploit the ecological opportunities at the furthest limits of the planet.

Heart normal anterior exterior anatomy. Photo credit: Patrick J. Lynch, medical illustrator – Patrick J. Lynch, medical illustrator
Heart normal anterior exterior anatomy. Photo credit: Patrick J. Lynch, medical illustrator – Patrick J. Lynch, medical illustrator

So how do migrants burn energy at 10x the rate of elite endurance athletes? Human lungs are tidal (breath in breath out), bird lungs are through-flow like jet-turbines. There is no dead-space in a bird’s lungs.


Hedenström A (2010) Extreme Endurance Migration: What Is the Limit to Non-Stop Flight? PLoS Biol 8(5): e1000362.

Sissel Sjöberg, Gintaras Malmiga, Andreas Nord, Arne Andersson, Johan Bäckman, Maja Tarka, Mikkel Willemoes, Kasper Thorup, Bengt Hansson, Thomas Alerstam, Dennis Hasselquist. Extreme altitudes during diurnal flights in a nocturnal songbird migrant. Science, 2021; 372 (6542): 646 DOI: 10.1126/science.abe7291

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McWilliams, S. R., and Karasov, W. H. (2005). “Migration takes guts: digestive physiology of migratory birds and its ecological significance,” in Birds of Two Worlds, eds P. P. Marra and R. Greenberg (Washington, DC: Smithsonian Institution Press, 67–78.

Mathot KJ, Kok EMA, Burant JB, Dekinga A, Manche P, Saintonge D, Piersma T. 2019 Evolutionary design of a flexible, seasonally migratory, avian phenotype: why trade gizzard mass against pectoral muscle mass? Proc. R. Soc. B 286: 20190518.

JUNE 4, 2021