A deer horn, or deer whistle, is a sonic generator (whistle

Whistle

A whistle or call is a simple aerophone, an instrument which produces sound from a stream of forced air. It may be mouth-operated, or powered by air pressure, steam, or other means...

) mounted on automobiles intended to help prevent collisions with deer

Deer

Deer are the ruminant mammals forming the family Cervidae. Species in the Cervidae family include white-tailed deer, elk, moose, red deer, reindeer, fallow deer, roe deer and chital. Male deer of all species and female reindeer grow and shed new antlers each year...

. A Shu-Roo is a similar device used in Australia to scare off kangaroos. The device produces ultrasound

Ultrasound

Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...

, and relies on deers' allegedly acute sense of hearing. 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.

Scientific studies of these devices have indicated that they do not in fact reduce collisions.

Researchers with the Georgia Game and Fish Department and University of Wisconsin–Madison

University of Wisconsin–Madison

The University of Wisconsin–Madison is a public research university located in Madison, Wisconsin, United States. Founded in 1848, UW–Madison is the flagship campus of the University of Wisconsin System. It became a land-grant institution in 1866...

 have pointed out several reasons why these devices do not (and probably could not) 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.
• We know little about the auditory limits of deer, but what we do know indicates that deer hear approximately the same frequencies as humans, and thus if we can't hear it, they probably can't either.
• If deer could hear ultrasound, we do not know that it would alarm them or induce a flight response
  Fight-or-flight response
  The fight-or-flight response was first described by Walter Bradford Cannon....
  
  . Certainly if they heard it on a regular basis, they would get used to the sound and not react to it.

In addition to the Georgia and Wisconsin studies, research by the Washington State University as well as a study by the Ohio State Police Department also indicated the whistles are ineffective. Independent testing carried out by the Department of Zoology at the University of Melbourne

University of Melbourne

The University of Melbourne is a public university located in Melbourne, Victoria. Founded in 1853, it is the second oldest university in Australia and the oldest in Victoria...

 reached the same conclusion about shu-roos and kangaroos.

These results are summarized in the following articles:

• Insurance Institute for Highway Safety
  Insurance Institute for Highway Safety
  The Insurance Institute for Highway Safety is a U.S. non-profit organization funded by auto insurers, established in 1959 and headquartered in Arlington, Virginia. It works to reduce the number of motor vehicle crashes, and the rate of injuries and amount of property damage in the crashes that...
  
   Advisory #31
• Whistles and Whitetails
• An article from Road Management and Engineering Journal

See also Shu-Roo.