What Limits the Size of Birds?

Kori Bustard in Etosha Namibia (photo by Winfried Bruenken, published at Wikipedia)
Kori bustard in Namibia (photo from Wikimedia Commons)

29 December 2009

What limits the size of flying birds? Why are there no behemoths like whales or elephants?

Weight is a limiting factor but it’s not the whole story.  The flightless ostrich weighs up to 300 pounds but our largest airplanes weigh more than 750,000 pounds and they can fly.

The answer is a combination of both weight and flight feathers.  Sievert Rohwer and his colleagues at the Burke Museum found that as a bird’s mass increases its flight feathers must be longer to carry its weight.  However the longer the flight feathers are, the longer it takes for them to grow.

At the high end of body mass the primaries grow so slowly that they’re in danger of wearing out before they can be replaced.  This causes large birds to either molt very slowly – sometimes over a period of years – or, in the case of geese, to lose all their flight feathers at once and hang out in the water until the feathers grow back.

The heaviest flying bird is the kori bustard, pictured above.  Native to the African desert he weighs up to 44 pounds, almost twice the weight of North America’s largest bird, the trumpeter swan, at 23 pounds.

Southwestern Pennsylvania’s heaviest birds are even smaller.  A large male wild turkey weighs 16.2 pounds, a large male Canada goose weighs 9.8 pounds.  My beloved peregrines, though fierce, are lightweights.  The male typically weighs 1.5 pounds, the female 2.2 pounds.

And just because a bird is large doesn’t make him heavy.  The wandering albatross has the longest wingspan at 8.2 to 11.5 feet (up to twice a man’s height) but weighs only 13-26 pounds.

So if a bird wants to fly it can’t weigh much more than 44 pounds — and that’s stretching it.   As you can see, the kori bustard spends a lot of time walking.

For more information see this article in Science Daily or the journal at PLOS Biology.

(photo of a kori bustard by Winfried Bruenken, published under Creative Commons license on Wikipedia.  Click on the photo to see the original.)