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Why do migratory birds build up fat before beginning a long flight?
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Migration
A herd of common wildebeest (Connochaetes taurinus) migrating across a dusty savanna in Africa. The animal is a keystone species (i.e., a species with a disproportionately large effect on its biological community) in plains and acacia savanna ecosystems from southeastern Africa to central Kenya.
Migratory birds accumulate fat in their body tissues prior to migration to provide the necessary energy required for their long-distance flights.12
This process is part of a broader shift in metabolism controlled by the bird's endocrine system, specifically the pituitary gland. As environmental cues—such as changes in day length—signal the approach of the migration season, the pituitary gland secretes hormones that alter metabolic patterns. This hormonal shift triggers an increase in food consumption and the subsequent storage of fat, which serves as a vital fuel reserve for the strenuous journey. These physiological preparations ensure that the animal is physically equipped to sustain the energy demands of its migratory flight.12