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Why can a sudden autumn cold spell cause some birds to leave immediately?

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A herd of wildebeest, including calves, moves across a dusty savanna in Africa with scattered trees visible in the background. — 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.

A sudden autumn cold spell can trigger the immediate departure of many migratory birds because these environmental conditions serve as a critical cue to initiate flight.12

While internal physiological changes—such as the accumulation of fat in body tissues—prepare a bird for the journey, these processes are primarily governed by the pituitary gland in response to factors like decreasing day length. However, because ecological conditions are often irregular, relying solely on internal rhythms would be risky for survival. Consequently, birds use external environmental cues to determine the precise timing of their departure.12

For many species, temperature and weather patterns act as the final "go" signal. A sharp drop in temperature often signals the onset of harsher conditions or a decline in food availability, prompting birds to leave immediately to avoid the risks associated with staying in an area that can no longer support them. Sensitivity to these weather cues varies significantly between species, with some being highly dependent on temperature and barometric pressure to time their movements.21