The physical and chemical processes that maintain a bird's life are called, collectively, its "metabolism." A flow of energy is required to run the metabolism of any organism, and the basic energy source for all birds is the sun. Green plants "capture" the sun's energy in the process of photosynthesis, and birds then acquire it by eating plants or by eating other animals that eat plants. The energy is used to do the work of building tissues, contracting muscles, manufacturing eggs, processing information in the brain, and powering all the other activities of a living bird.

The entire metabolic process is run by biological catalysts known as enzymes. They are long, chain-like protein molecules that are twisted into characteristic three-dimensional shapes. Enzyme molecules function rather like templates to hold reacting molecules together in the proper position to speed their interactions. If your enzymes lose their shape ("denature"), they stop functioning, your metabolism ceases, and you're dead. Birds are no different. That's why boiling kills; it denatures enzymes.

To compare rates at which different animals use energy, scientists calculate for each the rate at which a resting animal under no stress consumes oxygen. That consumption is then used to calculate the basal metabolic rate, which is expressed as the number of kilocalories of energy used per kilogram of body weight, per hour. Small birds have proportionately larger surfaces (through which heat is lost) in relation to their mass of metabolizing tissue than do large birds. A Bushtit can maintain a body temperature like a Tundra Swan's because it has such a higher basal metabolism (i.e., uses proportionately more energy). Hummingbirds, with their tiny bodies and high levels of activity, have the highest metabolic rates of any animals -- roughly a dozen times that of a pigeon and a hundred times that of an elephant. To maintain those rates, hummers have to consume about their weight in nectar daily. In fact, a warm-blooded animal can't be smaller than a hummer or a shrew. Further reduction in size would make it impossible for the creature to eat fast enough to maintain its body temperature.

The basal metabolic rates of nonpasserine birds are very similar to those of some mammals. Passerines, however, tend to have 30-70 percent higher metabolic rates than either nonpasserines or mammals, for reasons that are not understood. Birds do not generally use more energy than mammals to get the same job done -- indeed they often use less. Flying is faster and energetically cheaper than walking or running for comparably heavy animals. But, overall, birds and mammals are metabolically very similar.

When they are active, birds, of course, have metabolic rates wen above their basal metabolism. When hovering, hummingbirds are using energy at as much as eight times the resting rate. At the other extreme of their activity range, hummingbirds may become torpid at night -- that is, they let their body temperature drop, often until it is close to that of the surrounding air. A torpid individual may have a temperature 50 degrees F below its normal 104 degrees F, and a metabolic rate a third that of the basal metabolism. Generally, the temperature of torpid individuals is regulated at a level which may be correlated with the environment, being higher in tropical than in temperate zone species.

Hummingbirds do not become torpid every night. The ability to "lower their thermostats" appears to have evolved as a device for conserving energy, as when surviving periods of food shortage. At their active metabolic rate, hummers are only a few hours from starving to death; periods of bad weather threaten them severely even at their basal rate. Some other birds, such as swifts and poorwills, can also become torpid, but their lowered metabolic states have not been as thoroughly studied as those of hummingbirds.

When you are observing hummingbirds, you may see hummingbird (sphinx) moths hovering around flowers and sucking nectar through their long tongues. The parallels between the behavior of these day-flying moths and the hummers are striking. The larger of the sphinx moths are, in fact, heavier than the lightest birds. Interestingly, both birds and moths operate at similar body temperatures when hovering and feeding; the moths use metabolic heat generated by vibrating their wing muscles to raise their temperature to as high as 104 degrees F The "warm-blooded" (endothermic) birds drop their temperature during the night (when they are at rest) to conserve energy. The "cold-blooded" (ectothermic) sphinx moths become endotherms and use metabolic heat to raise their temperature only when they must to reach operating temperature for flight.

In cold weather, all nontorpid birds must operate at well above their basal metabolic rates in order to maintain their body temperatures. Small species, such as Black-capped and Boreal Chickadees that overwinter in temperate and subarctic areas, are at particular risk of freezing. They have proportionately large surface areas through which to lose heat and thus must eat continuously during short daylight hours to stoke their metabolic fires. If they do not, they will not reserve enough energy to see them through the long night. A wintering chickadee living at forty degrees below freezing must spend something like twenty times as much time feeding per day as it would in the warmth of spring.

Birds have only slightly higher body temperatures than mammals; avian temperatures range from around the human level of 98.6 degrees F (penguins, Whip-poor-wills) to 104 degrees (most resting birds). But in general, the temperature ranges of the two groups, like their overall metabolisms, are remarkably similar, considering their different modes of life. Both have evolved to function at temperatures just below those at which the crucial protein enzymes begin to lose their stability, change their shape, and cease to function (denature). Maintaining constant body temperature is thus not just a problem for birds trying to keep from chilling in cold weather; it is an even more critical problem when the air temperature rises above that of the body. Then birds must avoid overheating and sudden death. The relatively large body surfaces of small birds take in environmental heat (and lose cooling water) quickly. That is one reason few songbirds are evident at midday during heat waves; they seek shade and become inactive. Soaring birds, in contrast, may take advantage of thermals -- rising packets of warm air -- to avoid midday heat and the denaturation of their proteins in the cool air of high altitudes.

Why do birds (and mammals) run these risks of maintaining a high, constant temperature, especially since it costs them to do so? A small bird must consume many times more food than an ectothermal lizard of the same weight that warms to operating temperature in the sun and cools again at night. One obvious reason for the constancy of their temperatures is so that birds and mammals can be active at night and during cold weather -- they can penetrate areas and take on activities from which reptiles are barred. No had could feed alongside a Boreal Chickadee in winter. Another advantage to constancy is that the thousands of temperature-sensitive reactions that compose the metabolism can be better coordinated if they are in a relatively uniform thermal environment.

But why are temperatures of endotherms (and ectotherms when they are active) so close to the point of overheating? High temperatures, besides increasing the rate of chemical reactions, permit important physical functions that depend on diffusion to go on more rapidly. Heat speeds the diffusion of transmitter chemicals in nerve connections; the hotter a bird can be, the more rapidly vital information can be processed and commands sent to the bird's muscles. This allows birds to react more quickly. So high operating temperatures have clear advantages for both avian predators and prey; and unlike hands and other ectotherms, birds are not dependent on the sun's warmth to attain those temperatures. It has also been suggested that maintaining a constant high brain temperature aids memory and facilitates learning.

SEE: Temperature Regulation and Behavior; Drinking; Hummingbird Foraging Bouts; Spread-Wing Postures.

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.