Autumn Migration Speed of Juvenile Reed and Sedge Warblers in Relation to Date and Fat Loads

Abstract

We analyzed speed of migration in two congeneric warblers, the Reed and the Sedge Warbler, Acrocephalus scirpaceus and A. schoenobaenus. Sedge Warblers migrated at a higher speed than Reed Warblers. The two species showed similar rates of fat deposition at our Swedish study site, although Sedge Warblers departed with lower fat loads. The higher speed of migration in Sedge Warblers and their lower departure fat loads suggest that they encounter stopover sites which offer higher relative fat deposition rates farther south. The amount of visible fat at the time of banding was positively related to the speed of migration. Estimates of speed of migration for the two species suggest that the recoveries were situated on average 76-111 km farther south per increase in fat score, corresponding to 58-85% of the expected distance a bird can cover by using the fuel of one unit of fat score.