Deer horn

A deer horn, or deer whistle, is a whistle mounted on automobiles intended to help prevent collisions with deer. Air moving through the device produces sound (ultrasound in some models), intended to warn deer of a vehicle's approach. Deer are highly unpredictable, skittish animals whose normal reaction to an unfamiliar sound is to stop, look and listen to determine if they are being threatened. If the whistle gives them advance warning, they may freeze on the roadside, rather than running across the road into the path of the vehicle.

In Australia, a different product, with electrically powered speakers (Shu Roo), is used to decrease collisions with kangaroos.

Researchers with the University of Wisconsin–Madison measured three devices and a press report said they found these three devices produced "low-pitched and ultrasonic sounds at speeds of 30 to 70 miles per hour; however, researchers were unable to verify that deer responded to the sounds."

Researchers with the Georgia Game and Fish Department have pointed out several reasons for ultrasound devices not to work as advertised:

Some deer whistles do not emit any ultrasonic sound under the advertised operating conditions (typically when the vehicle exceeds 30 mph).

Ultrasonic sound does not carry very well. It does not travel a long enough distance to provide adequate warning, and also is stopped by virtually any intervening object, so any curves in a road will block the sound.

Little is known about the auditory limits of deer, but current knowledge indicates that deer hear approximately the same frequencies as humans, and thus if humans can't hear a sound, deer probably can't either.

If deer could hear ultrasound, it is unknown if it would alarm them or induce a flight response.