Fat deposition and theoretical flight range of small autumn migrants in southern Germany

From 1987 to 1989, 11 passerine species caught during their autumn migration on the Mettnau peninsula at Lake Constance (SW Germany) were examined for weight, visible fat deposition and actual fat load using a Soxhlet extraction. From the amount of fat deposited, mean theoretical flight ranges were calculated. Visible stored fat is only deposited in large quantities towards the end of the migration period. Before migration, the mean fat content stored in relation to fat-free dry weight ranges from 9–12%, whereas during migration it ranges from 13–30%. There is no difference between long- and short-distance migrants. A weight change cannot be accounted for exclusively by fat deposition. The mean weight of first-captures was low in all species. Only 5 of the 11 species studied gained weight more or less extensively during the migration period. All species showed an increase in fat deposition during migration. Since 1972, a marked annual decrease in average body weight during migration was found in the Reed Warbler. On average, water content in relation to fat-free dry weight is between 208 and 254% during migration and between 216 and 270% during the premigration period. For the species studied at Mettnau, mean theoretical flight range is 49 to 85 km during the pre-migration period and 84 to 216 km during migration (n = 211 birds). An average flight range of up to 100 km implies short migrational ‘legs’ from Lake Constance to the nearby Swiss midlands, which is probable if only parts of the small fat reserves are utilized during their flight. Because birds have to increase their initial fat stores considerably to reach and cross the Mediterranean, a low fat deposition rate may lead to problems before they reach their winter quarters. Short flight ranges are discussed in connection with trends of decreasing populations in central Europe.