How does the Arctic tern (a sea bird) fly more than 80,000 miles in its roundtrip North Pole-to-South Pole migration? How does the Emperor penguin incubate eggs for months during the Antarctic winter without eating? How does the Rufous hummingbird, which weighs less than a nickel, migrate from British Columbia to Mexico? These physiological gymnastics would usually be influenced by leptin, the hormone that regulates body fat storage, metabolism and appetite. However, leptin has gone missing in birds - until now.