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The pied avocet is a black and white bird with a very long, upturned beak. It lives and breeds in the coastal areas of Europe and Central Asia. In winter, the bird moves to warmer areas - to Africa or South Asia.
The avocet uses its bill to search for food. It puts its bill into the shallow water and sweeps it from side to side to scare up little animals and catch them.
When feeling threatened the pied avocet utters a series of sounds and slightly varies the pitch level. It sounds like the pied avocet approaches its enemy much faster than it really does. With a bit of luck, this will scare its enemy away. This sound effect is called the Doppler effect.
Just think of a fire truck siren. As long as the truck is approaching you, the siren sounds higher. When the truck is driving away from you, it sounds lower. This phenomenon is related to the sound waves. As long as the car is approaching it kind of pushes the sound waves ahead and compresses them, so that they sound higher.
