Most folks understand that metabolism has something to do with energy, but they are not sure what else metabolism means. Simply put, metabolism is the physical and chemical processes that maintain a bird’s life. A flow of energy is required to run and maintain 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 called photosynthesis, and the birds then acquire it by eating plants or by eating other animals or insects 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 whole of the 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 enzymes lose their shape (“denature”), they stop functioning, metabolism ceases, and death follows. Birds operate within these parameters. This is why boiling kills; it denatures enzymes. (When making nectar, it is always best to bring your water to boiling or near boiling to kill any potentially harmful bacteria and to retard fermentation.)
To compare the rates at which different birds use energy, scientists calculate for each the rate at which a resting bird 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 larger birds. Hummingbirds, with their tiny bodies and extremely high levels of activity, have the highest metabolic rate of any bird or animal in the world (one species of shrew is thought to have a metabolic rate as high as or higher than a hummingbird). For example, a hummingbird has a metabolic rate roughly twelve times that of a pigeon and one hundred times that of an elephant. To maintain those rates, hummers have to consume about their own weight in nectar daily. (During migration, this figure can increase exponentially.) In fact, a warm-blooded animal cannot be smaller than a hummingbird or shrew. Further reduction is size would make it impossible for the critter to eat fast enough to maintain its own body temperature.
When they are active, birds have metabolic rates well above their basal (resting) metabolic rate. When hovering, for instance, they 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 hummer may have a temperature 50 or more degrees F below its normal range of 104 – 106 degrees F and a metabolic rate a third that of their basal metabolic rate. Generally, torpid individuals regulate their temperature at a level 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 shortages or extremely cold weather. 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 rates are not as thoroughly studied as those of hummingbirds.
Maintaining constant body temperature is 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. Birds must then avoid overheating at all costs and the sudden death that can result. 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 denaturizing of their proteins in the cool air of high altitudes.
Birds have high body temperatures so they can be active at night (like poorwills and nighthawks) or during colder weather, thereby being able to take advantage of otherwise inaccessible areas and/or take on activities from which others are barred. A constant body temperature also enables their metabolism to function better in a relatively uniform thermal environment.
Higher body temperatures increase the rate of chemical reactions, permitting 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 is, the more rapidly vital information is processed and commands sent to the bird’s brain and muscles. This allows the bird to react more quickly. Therefore, high operating temperatures have clear advantages for both avian predators and prey; and unlike lizards and other cold-blooded animals, birds are not dependent on the sun’s warmth to attain those temperatures. Maintaining a constant high brain temperature aids memory and facilitates learning.
Much of this information comes from The Birder’s Handbook by Ehrlich, Dobkin and Wheye. This is an essential reference source that we consult often and try to keep in stock in both stores. Packed with almost 800 pages of natural history information of North American birds, it is a bargain at $20.00.