Hazards of Being a Bird
from the book, "Petrified Lightning"
Hazards of Being a Bird
The basic problem of a bird's lifestyle is maintaining a temperature as high as 104 degrees Fahrenheit. Because birds are warm blooded (homeothermic), getting enough food is an urgent and ever—present need. Birds that spend much time in flight are relatively small and lose body heat more rapidly than heavier ones do. So they are notoriously voracious: compared with the elephant that eats 2 percent of its weight per day, the hog 4 percent, and the cold—blooded crocodile a mere 0.7 percent, a bird will eat up to 30 percent of its weight every day.
The law of surface—size—to—body—weight ratio has almost caught up with the hummingbird. This tiny bird is at the lowest limit of size at which a warm—blooded creature can exist. The hummingbird's heart takes as much space as it can afford and still leave room for other essential organs. Compared with the human heart, which is 0.42 percent of total body weight, the hummingbird's heart is 2.37 percent and beats at the awe—inspiring rate of 615 times per minute. The bird has learned one desperate expedient: it hibernates every night. In this condition of dormancy its body temperature goes down almost to that of the environment, and its need for oxygen dips correspondingly. If the hummingbird's ancestors had not evolved this function, it would starve to death overnight.
Several other species of birds can, when food is scarce, become temporarily cold—blooded and dormant. By reverting to the cold—bloodedness of their ancestral reptilians, the nestlings of the European swift can survive at least 10 days of complete fasting, an unusual ordeal for a young and growing bird. Another hibernator is the whippoorwill. Its body temperature drops from a normal 102 degrees to 65 degrees Fahrenheit, its breathing stops, and its digestive processes cease. This "false hibernation" can continue for several months until food is again available.
Most birds avoid the problems of winter cold by migrating southward. Those that do not must develop some means of maintaining warm—blooded body temperatures in the face of a rough winter. Freezing rain, for example, is as dangerous to birds wintering in the north as it is for bush pilots in their single—engine planes. During the night, freezing rain can fasten the wings of chickadees and juncos to their backs. At dawn when they try to fly, the birds fall helplessly to the snow and most likely become the next meal for a predacious hawk or eagle.
Some wintering birds such as the grouse and partridge are able to detect when a severe arctic storm is coming. They escape by flying full speed into a bank of soft snow, thus insulating themselves against the windchill and burying themselves without a telltale mark for predators. Occasionally they would be better off dealing with the swift death handed out by a predator, because their refuge may be iced over by freezing rain and the sanctuary becomes a tomb.
Birds that have evolved the ability to survive the hostile Arctic winter—discounting the fate of some icebound grouse, partridge or chickadee—face the hardship of finding adequate food in an environment where food is ever so scarce. But those that habitually migrate south for the winter do not find a bed of roses either. Migration is the great adventure for the majority of bird populations, but it also involves their greatest risks. Just why birds migrate is not completely explainable, but without doubt the choice to fly off results from transcendental memory (instinct) and not some individual or group decision.
Among the travelers from the northern hemisphere are the 15 species of wild geese that make formidable journeys southward each fall to their wintering ranges and return north as far as the Arctic to their breeding grounds. Each population follows a specific flyway hundreds or thousands of miles each way; one species crosses the Himalayas.
The wild goose does not waste precious energy reserves in elaborate courtship rituals. A pair of geese engage in a simple neck—stretching dance at the wintering grounds, honk in mutual acknowledgement, and mate for life. Considering that two compulsive, devoted parents are necessary to hatch, raise, and educate a brood of goslings, monogamy and fidelity are the only efficient and ecologically sound behaviors.
As they prepare to fly north to the breeding grounds, wild geese are unable to store fat reserves as do the smaller birds; extra weight would jeopardize their ability to become airborne. Therefore the goose must constantly be feeding (and emptying) itself during the flight home. Fortunately, the long hours of daylight as they follow spring northward and the plentiful supply of young, nutritious food make the journey adequately successful.
Back on the breeding grounds the work really begins, as the female lays and incubates a clutch of eggs. The male keeps her nourished, so that both will be able to nurture, train, and protect the goslings. To have them ready for the great flight southward in the fall takes the best efforts of two geese, working full time. Where family values are concerned, the wild geese have no equal.