Every year, flocks of knots embark on a 10 000 km one way journey, from Siberia to West Africa, only stopping briefly enroute to refuel, arriving at their destination more than four days later and tens of grams lighter. How the migrants fuel this incredible feat of endurance puzzled scientists for years until an international team from Sweden, Switzerland and The Netherlands began monitoring red knots' metabolism during endurance flights in a wind tunnel. Susanne Jenni-Eiermann and her colleagues traced the levels of fuel metabolites in migrating bird's blood over the first ten hours of the long haul flight, expecting to find a long transition from carbohydrate metabolism to energy-rich lipids. But instead they discovered that the red knots quickly switch to a high-energy mixture of lipid and protein metabolism, which they maintain for the duration of the flight(p. 2453).
As knots prepare to embark on their annual journey to warmer climes, they lay in stores of fat and protein, ready to consume the lot during long migartion flight. Unlike mammals, which use carbohydrate and lipid fuels to generate energy, birds rely on a high-energy cocktail of lipids and protein. Jenni-Eiermann and Lukas Jenni had only been able to estimate which fuels the birds consumed by analysing the bird's metabolites as they stopped off briefly on their long haul flight. To get a real handle on the bird's fuel consumption, they needed to continuously monitor birds over the flight's duration. Not an option in the wild.
But through a long standing collaboration with Åke Lindström in Sweden, Jenni-Eiermann had access to a wind tunnel in Lund. Lindström's group could monitor the birds flying for hours at a time, collecting blood samples as they cover hundreds of kilometres, without ever leaving the lab!
Theunis Piersma collected twelve adult birds when they briefly stopped in the Dutch Wadden Sea en route from Siberia to Africa and sent them to Sweden,ready to fly in the wind tunnel. Lindström remembers that the knots flew perfectly in the wind tunnel. They became tame very quickly, and didn't seem perturbed when Lindström's team injected them or took blood samples after several hours of flight.
After months of patiently watching the birds fly and collecting blood,Lindström's team sent the precious samples to Switzerland, ready for Jenni-Eiermann to begin the laborious task of analysing the bird's metabolites.
Jenni-Eiermann looked for evidence of lipid metabolism by checking fatty acid and glycerol levels in the birds' blood. She also measured the amount of uric acid that the birds were carrying to find the level of protein the bird consumed during its long journey. She expected to find that the birds started off burning carbohydrate, switching over a matter of hours later to the lipid/protein cocktail. But to her surprise, the birds had converted to their high-energy mix within one hour of take off, and didn't vary it over the remainder of the flight.
Jenni-Eiermann also wondered if birds that consumed more energy during the flight topped up the lipid fuel with extra protein. But again the birds did something different. When Henk Visser measured the amount of energy each bird consumed during a ten hour flight, he discovered that the birds with a higher metabolic rate simply turned up both fuel pathways.
As Jenni-Eiermann points out, `birds are extremely well adapted to endurance flights' and this is just another example of their amazingly highly tuned metabolism.