How well a bird flies depends on how big its heart is. The best flyers, like hummingbirds that dexterously hover in front of flowers, have the largest hearts. But unexpectedly, soaring and gliding turns out to be almost as much work as flapping wings.
Previous research had suggested this, because sustained flight requires more aerobic power, which depends on heart size. The heart is like a carburetor pumping fuel into an engine: the bigger the heart, the more blood a bird can pump to its flight muscles.
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Hummingbirds have the biggest hearts for their body size, about three per cent of their mass. In contrast, a pelican’s heart is just 0.8 per cent of its mass.
When a hummingbird hovers in place, air doesn’t move past its wings to generate the lift needed to keep it aloft. Instead, it beats its wings in a figure-of-eight pattern up to 80 times per second to generate its own airflow, much like a helicopter.
This is energetically costly, says at the Austral University of Chile in Valdivia. “A helicopter uses a hundred times or more fuel than an airplane of the same size, because all the sustainability given by aerodynamics of wings is paid by the rapid hovering of the rotor.”
But most birds don’t fly this way. Some flap their wings up and down to provide thrust, like geese. Others, like eagles and woodpeckers, soar and glide on updrafts of hot air. Finally, some ground-dwelling birds, like pheasants, take occasional “short flights: short strong bursts of flapping flight.
Flapping away
In theory, birds using these less costly forms of flying don’t need the big hearts hummingbirds do. To find out, Nespolo and his colleagues grouped 915 bird species into the four flight categories and compared their heart sizes. They also mapped this information onto a tree showing the birds’ evolutionary relationships.
They found birds’ hearts evolved towards sizes that coincided with their preferred flight mode. The optimal heart size for hovering was 2.43 times higher than it was for flapping flight and 3 times higher than gliding and soaring. The difference between hoverers and short fliers was nearly twice that.
It’s surprising that flapping fliers had similar heart sizes to gliders and soarers, says at the University of Florida in Gainesville. She had expected soaring birds’ hearts to be more like short fliers’. “I would have assumed that flapping flight would have required a lot more energy than gliding and soaring.”
Based on the evolutionary tree, it’s likely that modern birds’ common ancestor did short flights. Early birds diverged quickly into short fliers and flappers, before flappers split into gliders and hoverers.
Journal of Experimental Biology
Read more: This bird has flown: Unravelling the mysteries of bird migration
