Hummingbirds are tiny birds that can stop in the air in front of flowers. Flying like this uses a lot of energy. A study found that hummingbirds use sugar from flower nectar very quickly.

Within 30 to 45 minutes, the new sugar can give almost all of their energy. At night, they cannot drink nectar, so they cool their bodies to save energy. One bird's body temperature fell to 3.26°C.

Hummingbirds can hover in front of flowers, but this way of flying uses a huge amount of energy. In fact, for their body size, small hummingbirds have one of the highest metabolic rates of any warm-blooded vertebrate. In a 2007 study, researchers gave hummingbirds sugar water with a special carbon marker and checked their breath.

While the birds were actively feeding and flying, the new sugar gradually became their main fuel, supplying almost 100 percent of their energy after 30 to 45 minutes. At night, when they cannot feed, hummingbirds enter a sleep-like state called torpor. In the Andes, one bird lowered its body temperature to just 3.26°C.

Hummingbirds live at the physiological edge. As the physiologist Suarez noted in 1992, small hummingbirds reach the highest mass-specific metabolic rates among warm-blooded vertebrates, and the delivery of oxygen and fuel to their flight muscles may approach the structural limits of vertebrate design. Hovering is especially costly, and the heart can reportedly exceed 1,000 beats per minute. Unlike humans, who typically store food before using it, hummingbirds can burn their meals almost immediately.

In 2007, Welch and Suarez fed Anna's and rufous hummingbirds cane-sugar solutions carrying a distinctive carbon isotope signature and analyzed their exhaled breath. During frequent feeding, the contribution of newly ingested sugar rose over 30 to 45 minutes until it supplied nearly all the fuel for hovering. Such a lifestyle creates a problem at night, when the birds cannot feed.

Their solution is torpor, a controlled drop in body temperature. In 2020, Wolf and colleagues measured 26 birds of six species at about 3,800 meters in the Andes; every species entered deep torpor. These studies, however, involved relatively small samples. One black metaltail cooled to 3.26°C, the lowest body temperature reported for any bird or non-hibernating mammal.