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Double-jointed Hawks and Convergent Evolution

Two different hawks have evolved an identical (and outlandish) ability
© Dave Curtis View Large

Crane Hawks of Central and South America and African Harrier-Hawks both have legs that bend forward and backward. Each bird’s wonderfully peculiar leg adaptation is completely original - it evolved all on its own - even though the end result is the same. Scientists call this convergent evolution.

Full Transcript



Double-jointed Hawks and Convergent Evolution

Written by Bob Sundstrom

This is BirdNote.

Two different hawks—that live on opposite sides of the world—have evolved an identical and outlandish ability.

[Crane Hawk call]
The first is the Crane Hawk, of Central and South America. It’s named after the tall, statuesque birds called cranes, because of its long, slim, gangly legs.

Unlike most birds’ legs, the Crane Hawk’s are adapted to bend both forward and backward. This rare trait — known as a reversible tarsus — is something we might call being double-jointed.

On the far side of the Atlantic there’s a second long-legged and double-jointed bird of prey: the African Harrier-Hawk.

[African Harrier-Hawk calls]

As hawks go, the two are only distantly related. But similar animals in similar environments can sometimes evolve similar traits. Each bird’s wonderfully peculiar leg adaptation is completely original - it evolved all on its own - even though the solution at the end of all that experimentation ended up being the same. Scientists call this process convergent evolution.

Both hawks are experts in capturing prey that other raptors find too elusive. Their long, double-jointed legs allow them to reach into crevices or holes, and swipe back and forth at odd angles to snatch their prey.

Two hawks. Two continents. And one amazing solution.

For BirdNote, I’m Mary McCann


Producer: John Kessler

Managing Producer: Jason Saul

Editor: Ashley Ahearn

Associate Producer: Ellen Blackstone

Assistant Producer: Mark Bramhill

Narrator: Mary McCann

Bird sounds provided by The Macaulay Library of Natural Sounds at the Cornell Lab of Ornithology, Ithaca, New York. Recorded by Alejandro Gutierrez Marquez, Jennifer F. M. Horne and David Guarnieri.

BirdNote’s theme was composed and played by Nancy Rumbel and John Kessler.

© 2019 BirdNote   August 2019