Hummingbirds require an enormous amount of energy to beat their wings fast enough to hover and maneuver. In many ways they appear to retain  some of the flight patters of insects, but they have an enormous amount of mass in comparison. Many changes in cell structure must occur to allow this high metabolism rate, and most specifically in mitochondria to be able to provide such large quantities of energy.

Quite a few changes in cell morphology and physiology might be expected to help deal with these large metabolism rates. Greater oxygen and carbon dioxide diffusion rates in the lungs would speed the movement of these gasses, as well as increased cardiac output and increase in capillary density. More white muscle fiber might be expected as they can produce more rapid movement, but hummingbirds can also sustain their flight longer then would be expected with these mitochondria poor fibers.

In the muscle cells you may also expect to find faster acting myosin bridges, and a slightly different affinity for Calcium ions that allows for faster contraction and relaxation of muscles to avoid tetanus in the muscles and allow for a fast twitch response. More Calcium pumps and channels would also help with the quick uptake and dispersal of the ion for activation of the muscle fibers and fast response time. Fibers would also be expected to be small and would be expected to have a large capillary profusion to carry nutrients and waste effectively. More 
carrier proteins for glucose and other nutrients would also seem necessary, or a higher dependence on high fat diet and metabolism due to being able to extract much more energy from fat then carbohydrates.

The mitochondria would be expected to be in greater number and have a high inner surface area with more cisterna in addition to having more oxidizing, electron transporting, and ATPase proteins. They would also be near to blood supply and be in great numbers.

In reality most of these differences are in effect in comparison to other mammals or 
birds. Hummingbirds have almost 10x the oxygen diffusion capacity of most mammals and heart rates of up to 1400 BPM. Their cardiac output is 5 times their body mass per minute and circulates completely in less then a second during hovering flight.

Their flight muscle fibers have  extremely high capillary perfusion, and high myoglobulin content. 
Mitochondria are in extremely high degree of clustering near capillaries and make possible substantially higher rates of oxygen flux then if  they were uniformly distributed though the muscles. The inner membrane surface area is extremely close to the theoretical maximum per unit volume for optimal metabolism. If it had much more there would be less room for the necessary enzymes to function. Evolution has given these birds near perfect capability to produce optimum amounts of energy and allowed them to achieve the highest known mass specific metabolic rates known in the vertebrate world.

There is a cost to all this energy production though. Even though it makes them suitable for long term hovering flight, their muscles would not be capable of heavy lifting or great strength. The hummingbird's metabolism and flight greatly limit the size of the bird. A large amount of carbohydrates must be ingested to sustain its energy needs. Most other organisms either need more power, or have sizes that make this high energy system poorly adaptable.

Hummingbirds also live in quite cold environments. As it grows cold they are able to survive through a mechanism called torpor which allows their body temperature to drop and then re-warm. An enzyme called mitochondrial uncoupling protein (UCP) is believed to cause this re-warming that allows the hummingbird to function. It is known that UCP uncouples respiration from ATP synthesis by short circuiting the inward proton flow, thereby generating heat. UCP is normally expressed during the colder months, and mostly in the heart and muscles of the birds.

Overall these birds have developed some amazing respiratory and oxidative mechanism that allows them to be a wonder among the animal kingdom.

REFERENCES:

Gene expression, tissue distribution and potential physiological role of uncoupling protein in avian species; Sami Dridi et al; Comparative Biochemistry and Physiology - Part A: Molecular&Integrative Physiology; Volume 139, Issue 3, November 2004, Pages 273-283

Adaptive  thermogenesis in hummingbirds; José Eduardo P. W. Bicudo1,*, Antonio C.  Bianco2 and Cláudia R. Vianna1,3; The Journal of Experimental Biology  205, 2267-2273 (2002)

The Quest for Speed: Muscles Built for High-Frequency Contractions; Lawrence C. Rome and Stan L. Lindstedt;  News Physiol Sci 13: 261-268, 1998; 1548-9213/98

Mitochondrial  respiration in hummingbird flight muscles; R K Suarez, J R Lighton, G S  Brown, O Mathieu-Costello; PNAS June 1, 1991 vol. 88 no. 11 4870-4873