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Running dry
Before setting off, they put on just enough fat for the journey, so once the fat is used up, they know they must be there. It’s like filling a car with just the right amount of petrol for your journey and then waiting until the tank runs dry. Of course it helps if you set out in the right direction! Once the fat is gone and the birds have returned to their normal body weight, they lose any urge to migrate further.
Last modified: 11 January 2005