Stopover Ecology of Autumn Migrating Bluethroats Luscinia s. svecica in Relation to Age and Sex

The ecology of autumn migrating Bluethroats Luscinia s. svecica of different age and sex classes was investigated at a stopover site in eastern Sweden, and data were used to test predictions from a model for optimal bird migration. Adults and juveniles migrated simultaneously but the migration period of adults was more concentrated. Adults carried more fat and stopped over for a shorter time than juveniles. In contrast to juveniles, adults did not lose mass after arrival and started accumulating fat earlier than juveniles. Adult birds can thus be regarded as more efficient migrators, a situation consistent with recent data showing that adult Bluethroats migrate at a higher speed than juveniles. Among juveniles, females (being smaller than males) arrived on average three days before males. If females have lower resourceholding potentials than males, the time difference could be due to an allocation of the migration time by females in order to avoid competition. However, no differences in fat deposition patterns or stopover times were found between sexes. The stopover data at least partly support the time-minimization model for optimal bird migration.