Why hummingbirds are so big-hearted

BIRDS fly better if they have big hearts. The best flyers, like hovering hummingbirds, have the largest. When a hummingbird hovers, it beats its wings in a figure-of-eight pattern up to 80 times per second, much like a helicopter. This is energetically costly, says Roberto Nespolo at the Austral University of Chile. But most birds don’t fly this way. Some flap their wings up and down, like geese. Others, like eagles, soar on updraughts of hot air, while some ground-dwellers, like pheasants, undergo only short bouts of flapping.
In theory, birds using more costly forms of flying should have larger hearts. The bigger the heart, the more blood a bird can pump to its flight muscles. To find out, Nespolo and his team grouped 915 bird species by flight type and compared their hearts. Hummingbirds had the biggest hearts for their body size, at 3 per cent of their mass. In contrast, a pelican’s heart is just 0.8 per cent of its mass. The sizes of birds’ hearts matched their flight mode. The optimal size for hovering was 2.43 times that for flapping and 3 times higher than gliding (Journal of Experimental Biology, DOI:10.1242/jeb.162693). It is surprising that flapping fliers had similar hearts to gliders, says Rebecca Kimball at the University of Florida. “I would have assumed that flapping flight would have required a lot more energy.”

-----NEW SCIENTIST DECEMBER 2018