Humans have been fascinated by flight since their earliest writings, and accounts of attempts to master the airways are scattered throughout history. Using kites and gliders, early pilots (sort of) conquered the air. Budding engineers watched birds and tried to emulate them, all the while missing two critical elements — a powerful engine and an extremely lightweight design.
In birds, power is developed by massive breast muscles that seem to spill out of the chest and require a large keel on the sternum to provide enough surface area for attachment. To support this power plant, birds have high metabolic rates, as well as high temperatures and a rapid heartbeat, all fueled by high-energy foods — resulting in a high-compression engine.
Adaptations for lightness are varied. Feathers, for example, are incredibly light (and unique to birds) and essential for flight. They provide excellent insulation and thereby help maintain high body temperatures — even in the extreme Arctic cold, where land mammals don’t tread. They also contribute to aerodynamic efficiency by helping form a teardrop body shape, augmenting lift, and they’re sufficiently flexible to bend and fan the air to create thrust.
Perhaps the most remarkable adaptations for lightness are found in the avian skeleton. It has been reported, for example, that the skeleton of a Great Frigatebird with a seven-foot wingspan weighs about four ounces, which is less than the total weight of its feathers.
Two important skeletal changes contribute to the low weight of birds. The first is hollow long bones —large, central cavities surrounded by a thin osseous layer. Inside the cavities of some of the long bones are bony strands arranged in a W-pattern, like bridge trusses. The strands maximize strength with a minimum of materials and weight. The second skeletal change is the structure of flat bones, such as those found in the skull, made of material that is loose and spongy rather than dense and compact.
The modern avian skull differs dramatically from those of the earliest known birds in the fossil record. Avian fossils have dinosaur-like skulls with thick jaws supporting many prominent teeth. The much smaller and lighter modern skull has thin jaws that lack teeth. The muscular avian gizzard compensates for the lack of teeth by grinding food, with the aid of ingested pebbles. The transfer of “chewing” to the gizzard also moves the center of gravity to a more favorable position under the wings, a distinct benefit for flight.
The efficiency of birds’ digestive and excretion systems also contributes to a lightweight design. They digest food much faster and more completely than other vertebrates, so they void a relatively small amount of fecal material. Such efficiency is important, because it prevents birds from carrying a lot of undigested food in their intestines. A thrush, for example, can defecate seeds 30 minutes after eating berries.
Birds also extract water from digestion and metabolism efficiently, so they can spend less time at the watering hole, and, therefore, reduce their exposure to predators. The diet of most birds is high in fat and carbohydrates. They can extract more water from these sources than from protein. But because birds do eat protein, getting rid of nitrogenous waste presents a special challenge.
All animals that consume protein have to deal with nitrogen by-products, which can be toxic. Mammals metabolize them to urea, which is somewhat toxic, and dilute it with water in the bladder until it can be voided as urine. Because of the weight concerns of birds, carrying around a water-filled bladder is not a good idea.
Birds, therefore, have developed a urinary system that metabolizes nitrogen waste products to uric acid, a non-toxic solid that appears as a white precipitate in urinary waste. Because uric acid does not need to be diluted with water, the urinary waste is relatively dry, which further conserves water. (I know, bird droppings that hit your windshield are anything but dry. Bird “droppings” that splatter our windshields also include watery waste from the large intestine.) Ureters drain urinary waste from the kidneys directly into a chamber in the back of the large intestine known as the cloaca, which opens directly to the outside. Consequently, birds lack urinary bladders and urethras, which further lighten the load.
Another adaptation for lightness is to keep testes and ovaries in a small regressed state, except during the breeding season. It has been reported that the testes of starlings increase by a factor of 1,500 as they enlarge for the breeding season. Since bird eggs are so large (the yolk is a single-celled ovum), there isn’t room for two functioning ovaries. So in most species, one ovary and oviduct has been lost (usually the right), and the birds function very nicely with one set.
There is no question that birds are masters of the sky. This is due to their aerodynamic shape combined with powerful, high-compression engines and exceedingly lightweight bodies. Despite the genius of current engineers, birds remain the ultimate ultralights, another example of those amazing birds.
Eldon Greij’s column “Amazing Birds” appears in every issue of BirdWatching magazine. This article appeared in the June 2017 issue. Eldon is professor emeritus of biology at Hope College, located in Holland, Michigan, and the founder of Birder’s World magazine.
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