Bird Fat (IMAGE) Penn State Caption These photos of the underside of white-throated sparrows show changes in fat deposition. Before migration, the birds can gain up to 50% of their body weight and then convert the fat to fuel their long flights. Credit Paul Bartell / Penn State Usage Restrictions Non Commercial use only License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.