We all recognize the Doppler effect when an ambulance siren rises in pitch as it speeds toward us, then drops as it recedes. (Click here for a car horn example.)

Here’s a bird that uses that sound effect.

American avocets have many techniques for protecting their nests from predators.  They pretend to incubate a fake nest, then walk a few steps and pretend again.  They distract the predator by walking toward him in a teetering tightrope walk with wings outspread.  And they mob aerial predators before they can reach the nests.

But the most amazing technique is reserved for ground predators.  When avocets swoop to chase them away they shout at them, modulating their pitch to resemble the Doppler effect.  This is done so convincingly that the predator thinks the bird is approaching much faster than it actually is.  Run away!

Tex Sordahl discovered this while studying American avocets and black-necked stilts in the 1970s and ’80s.  Both use the Doppler sound effect.  I’m sure he got a dose of it during his study.

(photo by Ingrid Taylar via Wikimedia Commons.  Click on the image to see the original.)