In broad brush terms, gular-fluttering in birds is the equivalent of sweating in humans. It is a strategy for reducing body temperature when weather conditions get too warm for the bird to be comfortable with normal breathing. For humans, the evaporation of the moisture generated by sweating helps to reduce body temperature. Birds have no sweat glands, so they have to adopt another strategy.
The strategy is to vibrate the skin of the throat. so that air is pumped over the moist skin, so that water evaporates and they keep cool. For a handful of species which do gular-fluttering, the speed of the vibration of the skin in the throat has been measured. This is done using high speed videos.
These flutter rates presented here come from a paper in the US journal Condor (Bartholomew GA. Lasiewski RC, Crawford EC 1968. Patterns of panting and gular flutter in cormorants, pelicans, owls, and doves). The species are all North American: Brown Pelican, 230 to 290 flutters per minute; Double-crested Cormorant. 645 to 730; Mourning Dove, 680 to 735; Horned Owl, 210 to 255; Bran Owl, 245 to 285 flutters per minute. For each species, there is a threshold temperature at which gular-fluttering starts. This is often in the range 35C to 40C. When the heat load is relatively small, gular-fluttering is intermittent. As the heat load increases, the proportion of time spent gular-fluttering increases. Finally, there is an upper threshold temperatujre, and once this is reached, often around 45C, gular-fluttering becomes continuous.