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In order to migrate successfully, animals must be capable of sustained movement for long periods of time. This movement requires considerable energy output, and different species of animals have evolved a variety of mechanisms to ensure sufficient body fuel for the trip. 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 10-cm- (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 800 km (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 3200 km (2000 mi) over water without landing.
In contrast, land mammals are able to graze on plants along the way as they travel, so they have no need for sizeable fat stores. In fact, it is important that these animals travel light so that they can remain agile to escape predators. Land mammals, such as African wildebeest, may walk more than 1600 km (1000 mi) when migrating.
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