I just read an interesting review published in Physiology of flight energetics and fuel use in nectar feeding hummingbirds and bats.
While flight is considered pretty high intensity exercise, hovering flight is even more demanding. Hummingbirds and nectar feeding bats are really tiny and thus have very high metabolisms to maintain body heat. Some of these animals are also migratory, which is an even greater energy demand. Remarkably, they sustain migratory flight using only the fat they stored prior to the start of their journey.
Nectar feeding hummingbirds and bats are able to quickly digest and utilize dietary sugars. This is because their digestive and cardiovascular systems are specially adapted to break down and move sugars from the digestive tract to where they need to go in the body very quickly. For longer flights, or during periods of fasting, birds fuel flight with fats they have stored. After feeding, studies have shown that hummingbirds quickly switch back to using the newly ingested carbohydrates. Their ability to use carbohydrates for energy only minutes after ingesting them is remarkable and much faster that sugar fluxes measured for humans and other mammals. Nectar bats appear to have a similar pattern of fat and carbohydrate usage.
What these observations mean is that flight in these nectar feeding hovering animals is determined by whether they are fasting or fed, which but not by the intensity of exercise. This is in contrast to mammals that use primarily fats at rest or low intensity exercise and switch to carbohydrates with high intensity exercise. In addition, skeletal muscles of hummingbirds and nectar feeding bats can use fructose and glucose equally well, which is in stark contrast to mammalian skeletal muscle that takes up mainly glucose for energy.
Welch KC, Myrka AM, Ali RS, MF Dick. The Metabolic Flexibility of Hovering Vertebrate Nectarivores. Physiology. 33(2): 127-137, 2018.