When birds feel the urge to migrate, nothing gets between them and their destination. Sustaining impressive metabolic outputs over lengthy periods of time, the birds cross seas, mountains and deserts while maintaining a 10 fold increase in their metabolic rate for days at a time. Even top human marathon runners clad in light running kit can only sustain an 8-9 fold increase for a matter of hours; well insulated birds on the other hand have to keep it up for much longer. How they dissipate the enormous amounts of heat generated by their muscles during migration puzzles Jacques Larochelle, and more precisely,how do they migrate across scorching deserts without succumbing to internal combustion? Intrigued by the animals' remarkable thermoregulation, Larochelle and student Jérôme Léger decided to test whether a migrating bird could shed much of its thermal load by radiative heat loss while traversing some of the world's hottest wildernesses(p. 103).
Surprisingly Larochelle's inspiration didn't come on a scorching summers'day. He realised radiative heat loss could have a significant cooling effect one clear autumn evening about thirty years ago; the roof of his car were covered in frost while the rest of the vehicle was ice free, even though the air temperature was a balmy 5°C. Larochelle recalls that at that time,`everyone thought that birds avoided flapping flight in air above 20°C',so he didn't make the association. It was only later when colleagues recorded birds migrating across the Sahara at 20°C that he recalled the car's frosty top. The car was losing an enormous amount of heat by radiation to the clear night sky heat sink. Could radiative cooling stop birds from boiling over while migrating at night too?
Testing his theory required some technical ingenuity. First Larochelle and Leger had to convince pigeons to lie in a flight position in a wind tunnel while they simulated the bird's exposure to mild flight winds. But how could they simulate the night sky's heat sink effect while generating a warm breeze in the small flight tunnel? Eventually the team decided to impregnate the wind tunnel's walls with combinations of ice, dry ice and methanol to plunge the walls to temperatures as low as a -78°C. Finally the team had to come up with a way of gently simulating the heat generated by the bird's labouring muscles. Knowing that physiotherapists use weak microwave radiation to gently warm damaged muscle, Larochelle carefully warmed the birds with weak microwaves to simulate the heat generated during a long flight. The team were ready to test whether the night sky was keeping the birds cool.
Monitoring the birds' wing feather temperatures with an infrared thermometer the team realised that the birds were much cooler than could be explained by convection. When the team dropped the wind tunnel walls'temperature 20°C below the ambient air temperature, it was clear that the birds could be losing as much as 50% of their heat by radiation alone. And when the team dropped the wall temperatures even lower, they realised that radiative cooling became so effective that the ambient air began warming the birds' outer feathers. So a clear night sky keeps birds cool when voyaging on sultry nights.