A male Lazuli Bunting (left), a male Indigo Bunting (right), and a female Indigo Bunting (below)
Males and females of nonmigratory species generally share “household chores” and look similar. In migratory species like buntings, males take on more territorial defense and are more brightly colored, and females do more of the nesting chores where drab colors are an advantage.
■ The bird respiratory system is fundamentally different from ours, and much more efficient. Instead of flexible lungs that expand and contract with each breath, birds have rigid lungs and air flows through continuously in one direction, back to front. Air flow and storage is managed by a system of air sacs, and breathing is controlled by the muscles of the rib cage. Because the lungs don’t move, the membranes for gas exchange can be thinner than in our lungs. This also allows intertwined tiny air passages and blood vessels to be arranged with a countercurrent flow (see this page), which transfers a lot more oxygen to the blood than in human lungs. It is thought that this breathing system evolved in dinosaurs more than 200 million years ago, at a time when the earth had only half as much oxygen as today, and birds now reap the benefit. Birds are essentially never out of breath, and if you see a bird panting after exertion it is because of overheating (see this page). In experiments, hummingbirds can still fly at an oxygen level equivalent to 43,000 feet elevation! That’s 50 percent higher than Mount Everest.
The respiratory system of a bird: the air sacs occupy a large part of the body of a bird, and some air sacs extend into the larger bones (not shown).
■ One of the most colorful birds in the world, the Painted Bunting is found in the southeastern United States from South Carolina to Texas. The adult male is an incredible rainbow of brilliant colors; females and immatures are a much plainer olive overall. The population nesting along the Atlantic coast of the southeastern U.S. is declining: one reason is the species’ popularity as a cage bird in Cuba, where they are trapped in the winter. Trapping them is illegal, but this is rarely enforced.
A male Painted Bunting
■ When the rib cage expands, air is pulled into the body. The expansion of the rear air sacs pulls fresh air in from outside, and the expansion of the front air sacs pulls fresh air forward through the lungs. When the rib cage contracts, air is forced out of the body. The front air sacs (holding “used” air) empty to the outside, while fresh air from the rear air sacs is pushed forward through the lungs. It is not known why air follows certain paths and not others. For example, inhaling causes the front air sacs to pull air through the lungs and not from outside. No evidence of physical valves has been found, and apparently it is simply the angles of connections in the airway that cause air to flow one way and not another.
A greatly simplified schematic diagram of a bird’s respiratory system, with air sacs in blue and lungs in purple. The air sacs expand to inhale (top) and contract to exhale (bottom), and fresh air always flows from back to front (right to left) through the lungs.