Q: Why do horses nicker?
A: Vocalizations are one of many means of social communication among horses. Horses tend to rely more on visual and other cues than on vocalizations. Vocalizations out of context are probably less informative or useful to the receiver horse without other meaningful visual or olfactory cues. It’s been suggested that vocalizations in horses are not likely to be transmitting very specific information but rather more likely providing social cues. Much more research could certainly be done, as the little we do know is so interesting.
People who have been around horses can probably all agree on what a nicker is, but here’s a definition from “The Equid Ethogram” by Sue McDonnell, PhD: “(A nicker is) a low-pitched, gutturally pulsated vocalization … the character of the nicker varies with the excitement of the situation.” Usually the horse’s mouth is closed but you see the nostrils moving. Nickers are generally short in duration (less than 2 seconds long), though you might hear multiple in succession. The volume will often rise and fall even within that short duration.
A nicker is commonly described as a companionable greeting or solicitous type of vocalization. This makes intuitive sense both because of its nature as a quiet sound, but also in the close-contact social situations in which it is typically heard: Between two friendly herdmates, between a mare and her foal, and between a stallion and a mare.
Horses also nicker toward a familiar handler or perhaps toward anyone doing familiar things that are reinforcing for the horse (for example, at feeding time). A study by Juarbe-Diaz and others found that mares that bonded with their foals were more likely to nicker than mares that rejected their foals (mares that rejected tended to squeal more).