Exhaustion
Both soaring and sailing birds are so proficient in aerial transportation that only recently have the principles been understood and imitated by aircraft pilots. The use of ascending air currents, employed by all soaring birds and easily demonstrated by observing gulls glide hour after hour along the windward side of a ship, are now utilized by man in his operation of gliders. Moreover, the whole structure of a bird makes it the most perfect machine for extensive flight the world has ever known. Hollow, air-filled bones, together with feathers, the lightest and toughest material known for flight, have evolved in combination to produce a perfect flying machine.
Mere consideration of a bird's economy of fuel or energy also is enlightening. The golden plover probably travels over a 2,400-mile oceanic route from Nova Scotia to South America in about 48 hours of continuous flight. This is accomplished with the consumption of less than 2 ounces of body fat (fuel). In contrast, to be just as efficient in operation, a 1,000-pound airplane would consume only a single pint of fuel in a 20-mile flight rather than the gallon usually required. Similarly, the tiny ruby-throated hummingbird weighing approximately 4 grams, crosses the Gulf of Mexico in a single flight of more than 500 miles while consuming less than 1 gram of fat.
One might expect the exertion incident to long migratory flights would result in arrival of migrants at their destination near a state of exhaustion. This is usually not the case. Birds that have recently arrived from a protracted flight over land or sea sometimes show evidences of being tired, but their condition is far from being in a state of emaciation or exhaustion. The popular notion birds find long ocean flights so excessively wearisome that they sink exhausted when terra firma is reached generally does not coincide with the facts.
The truth is, even small landbirds are so little exhausted by ocean voyages, they not only cross the Gulf of Mexico at its widest point but may even proceed without pause many miles inland before stopping. The sora, considered such a weak flyer that at least one writer was led to infer most of its migration was made on foot, has one of the longest migration routes of any member of the rail family and even crosses the wide reaches of the Caribbean Sea. Observations indicate that under favorable conditions birds can fly when and where they please and the distance covered in a single flight is governed chiefly by the amount of stored fat. Exhaustion, except as the result of unusual factors such as strong adverse winds, cannot be said to be an important peril of migration.