Silent mouth movements. Used by doves to converse when there is a potential threat nearby that could be attracted by noise or movement.
Also used between doves socially or privately. The mouth movements are chewing motions in groups of 2's and 3's to let the other know that they are ok or as a friendly gesture between a male and female introducing themselves or socially between any number of doves. It is used especially frequently between a dove couple that has eggs or babies to enable them to exchange communications without revealing the location of their nest. I believe that two mouth chews is the same as the friendly or non-hostile signal and three is an action type of motion such as a question of intentions.
If danger is near, then it can be several in a row to signal the presence of an imminent threat and used as a method to communicate silently without revealing their presence.
This action has no sound
Often the doves are seen carrying on a silent conversation between each other. They move their bill like they are talking and the other will answer the same way. They will also do this with a human. Three chews are usually a more action oriented meaning while 2 signal friendly or non-hostile intentions. A series of chews is imminent danger approaching.
The silent conversation allows the doves to communicate without alerting nearby threats or humans or even other doves or revealing their location. When a male and female greet for the first time, they often silent talk while sitting on the perch and then may progress to preening or nuzzling each other. The silent motions tell the other new dove that their approach is friendly. This also helps avoid having some aggressive second male who might have had his intentions set upon the available female from attacking the pair. Doves can be very jealous. By silently conversing and introducing themselves, the new pair avoids this type of confrontation.
This silent language seems to consist of even spacing and in patterns of 2's and 3's much like their verbal language which is also in 2 & 3 sound patterns. so their silent coded language contains some of the same messages when matched with similarly patterned sound meanings.