Dec 29 2009

What Limits the Size of Birds?

Published by at 7:38 am under Bird Anatomy

Kori Bustard in Etosha Namibia (photo by Winfried Bruenken, published at Wikipedia)
This is a question I never thought about until I read a brief article in Science Daily.  Then one thought lead to another, I opened up Google and I was off on a fact finding mission.

What is the heaviest bird that can fly?  We can’t count the ostrich – who can’t fly – even though he can weigh up to 300 pounds.  The heaviest flying bird is the kori bustard of the African desert, pictured above, who weighs 27 to 44 pounds.  At his heaviest this is almost twice the weight of North America’s largest bird, the trumpeter swan, who typically weighs 23 pounds.

Southwestern Pennsylvania’s biggest 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 small.  The male typically weighs 1.5 pounds, the female 2.2 pounds.  Yes, the female outweighs him.

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 now that we know the maximums, why do birds stop there?  Why are there no behemoth birds like whales or elephants?

The answer is that as a bird’s mass increases it takes longer for its flight feathers to grow.  This correlation was found by Sievert Rohwer and his colleagues at the Burke Museum at the University of Washington.

The bigger the bird the longer the flight feathers must be but the slower those feathers 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 on water until the feathers grow back.

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 the press release in Science Daily or the complete article in 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.)

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