- Birds migrate during the changing of the seasons to exploit distant food sources and avoid the physiological stress of cold climates. Some birds travel a few kilometers. Others travel across the globe. According to the Smithsonian National Zoological Park, the Arctic tern travels 30,000 kilometers from Arctic breeding grounds to Antarctic seas. It holds the record as the longest migratory flier.
- The ability of birds to fly long distances would not be possible without its complementary respiratory system, which is the most efficient of the animal kingdom. Together the two lungs and special air sacs constitute 20 percent of the bird's volume, compared to 5 percent in humans. The lungs of the bird always remain inflated, and the air sacs act as bellows to provide the lungs with a constant supply of oxygen.
- Migratory birds exhibit longer, pointed wings that minimize air resistance. They also have pectoral muscles that contain a preponderance of blood vessels and energy-producing units in the cell called mitochondria. Because migratory birds fly at such high altitudes where oxygen is more scarce---the bar-headed goose flies over the Himalayas at 29,500 feet---they have two unique adaptations: a high number of red blood cells and two specialized types of hemoglobin, rather than one.
- Migratory birds build fat supplies before leaving and burn most of it off during the trip. A recent New York Times article by Carl Zimmer reported pre-migratory adipose could represent as much as 55 percent of total weight. To accomplish this, they may switch to a diet of berries and fruits abundant in carbohydrates and lipids that are readily converted to fat. Fat storage can increase by as much as 10 percent per day, and the pectoral muscles become stocked with special proteins called enzymes that oxidize, or burn, fat brazenly.
- Transoceanic birds usually make a nonstop trip across entire oceans without any foraging breaks. Biologist Robert E. Gill Jr. found the bar-tailed godwit, which makes the longest nonstop flight of 7,100 miles in nine days, raises its metabolic rate between 8 and 10 times. By contrast, a professional bicyclist in the Tour de France could only raise his metabolic rate 5 times. Researchers Tim and Janet Williams say that if the blackpoll warbler was burning gasoline instead of body fat, it would reach the equivalent of 720,000 miles to the gallon.
- The internal behaviors that support migration are still somewhat of a mystery. Tests suggest migration and associated factors such as distance and timing are genetically inherited. Birds can leave their nests in response to environmental cues such as temperature or food supply and navigate based on magnetic fields, landmarks and olfactory senses. But the full methods have yet to be discovered.