Is long-distance bird flight equivalent to a high-energy fast? Body composition changes in freely migrating and captive fasting great knots

We studied changes in body composition in great knots, Calidris tenuirostri s, before and after a migratory flight of 5,400 km from northwest Australia to eastern China. We also took premigratory birds into captivity and faste d them down to their equivalent arrival mass after migration to compare org an changes and nutrient use in a low-energy-turnover fast with a high-energ y-turnover fast (migratory flight). Migrated birds were as economical as an y fasting animal measured yet at conserving protein: their estimated relati ve protein contribution (RPC) to the energy used was 4.0%. Fasted birds had an estimated RPC of 6.8% and, consequently, a much lower lean mass and hig her fat content for an equivalent body mass than migrated birds. Lean tissu e was catabolized from most organs in both groups, except the brain. Furthe rmore, a principal components biplot showed that individuals were grouped p rimarily on the basis of overall organ fat or lean tissue content rather th an by the size of specific organs. This indicates that organ changes during migratory flight are similar to those of a low-energy fast, although the l ength of the fast in this study probably accentuated organ reductions in so me functional groups. Whether the metabolic characteristics of a flying mig ratory fast follow the three-phase model described in many inactive fasting animals is unclear. We have some evidence for skeletal fat being cataboliz ed without phase 3 of a fast having been reached.