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How an animal migrates is significant in determining how much energy is needed as well as how much must be stored in the body. Flying, for example, is more physically intensive than walking or swimming, so migratory birds must build up large energy stores before they set off. Just before spring and fall migrations, certain birds increase their body fat—up to nearly 40 percent of body weight in some songbirds. Before migrating, the (4-in-) long ruby-throated hummingbird gains about 2 g (0.07 oz) of fat. This extra fat provides sufficient energy for this tiny migrant to fly (500 mi) from North America, across the Gulf of Mexico, to its winter home in Mexico. Some birds supplement this stored energy source with food along the way. Others make long, nonstop flights—the golden plover may travel (2000 mi) over water without landing.

The monarch butterfly, Danaus plexippus, is known for its extraordinarily long migrations. During the summer months, monarchs can be found throughout the continental United States and parts of Canada, and they migrate to the California coast and central Mexico for the winter. The longest recorded flight for a tagged adult is (1,800 mi). A large number of monarchs spend their winters in the mountains west of Mexico City. Scientists speculate that the mountainous climate provides a favorable mix of moist air and cool, but not freezing, temperatures. These conditions keep the butterfly from drying out and keep its metabolism low enough to conserve fat stores but high enough to maintain life.