No machine can be 100% efficient – and Hummingbirds (Selasphorus calliope) are no exception. As a result, when they flap their wings (typically at around 50Hz) they generate considerable quantities of heat. To find out how much, investigators at the Department of Biology, George Fox University, OR, and the Division of Biological Sciences, University of Montana, Missoula, MT, US, set up a series of experiments.
See: Powers D R, Tobalske B W, Wilson J K, Woods H A, Corder K R. ‘Heat dissipation during hovering and forward flight in hummingbirds.’ Royal Society Open Science, Dec. 2015, 2 : 150598.
Note: Lead author Dr. Powers, of the Powers Research Lab, has published a considerable body of work regarding hummingbirds.
Of related interest : Progress in Head Mounted Flying Animal Feeders