I came here wondering the same thing. A random google search turned up this: https://www.birdwatchingdaily.com/news/science/five-bird-species-that-make-the-lon

I came here wondering the same thing. A random google search turned up this: https://www.birdwatchingdaily.com/news/science/five-bird-species-that-make-the-longest-migrations/ https://www.birdwatchingdaily.com/news/science/five-bird-spe... > Researchers have learned that digestive tracts of long-distance migrants work overtime prior to migration. Food intake is increased, and the food is metabolized and stored as fat, which serves as the birds’ flight fuel. Bar-tailed Godwits store fat until it makes up about one half their weight. Then the digestive system shuts down so metabolic energy can be maximized for muscle use. Astounding.