Nature's Barometer

As I sit here at my computer observing the chickadees, pine siskins, and nuthatches that dart to and from the feeder outside my window, I can’t help but marvel at what truly remarkable creatures birds are. Avians occur in virtually every habitat on earth, from the frozen, windswept valleys of Antarctica to the humid rainforests of the tropics. Other than bats, they are the only vertebrate species that can fly. Some undertake amazing migration journeys. For example, the arctic turn travels more than 18,000 miles each year, flying from breeding grounds in the Arctic Circle to the Antarctic Circle where it spends several months at sea until the next breeding season.

The adaptations of bird physiology and anatomy are quite remarkable. For animals that fly, there is a distinct advantage to being lighter. Birds have several interesting tricks to lighten the load. For instance, their kidneys connect directly to their digestive system, so don’t need a urinary bladder, reducing their weight. Unlike mammals that have two ovaries, female birds have only one ovary, another strategy to minimize weight. Feathers are another adaptation for flight, and they are also a terrific form of light weight insulation that helps keep birds warm.

Perhaps one of the most interesting adaptations for flight involves birds’ skeletal and respiratory systems. In mammals, bones have a solid core made up of the gelatinous bone marrow, which is responsible for the production of blood cells. Birds also have bone marrow, but not in all of their bones.

Depending upon the species, most birds (but not penguins, loons, and puffins) have a hollow skull, humerus, clavicle, sternum, pelvic girdle, and lumbar & sacral vertebrae. These hollow bones are filled with air and have rigid internal struts that give them added strength. The air-fill bones are connected to the lungs through a series of air sacs located throughout the bird’s body.

These air sacs are quite extensive and actually have a larger volume than the lungs. Together with the air-filled bones, the air sacs make birds highly efficient at bringing in the large amount of oxygen muscles need to sustain flight.

Studies over the past decade or so have demonstrated that birds are very sensitive to changes in atmospheric pressure, which allows them to predict the weather much like a human meteorologist (only better!). The ability to sense slight changes in atmospheric pressure allows birds to detect a coming storm or a cold front. Knowing what the weather will be provides vital information that may help with migratory journeys or to trigger more intense feeding to “bulk up” for a coming snow storm.

The mechanism by which this happens is not well understood, but a leading hypothesis involves the air-filled bones and air sacs that are part of birds’ respiratory system. These internal structures are directly connected to the surrounding external atmosphere through the lungs. Sensing pressure changes is not unique to birds. You are able to detect a pressure change in your air-filled nasal sinuses when you ascend a mountain and feel your ears “pop.” It is thought that birds are much more finely tuned to pressure changes due to the extensive network of air-filled cavities throughout their bodies. In effect, birds are living barometers!