Abstract
The fat level and the body mass of local Great Reed Warblers in the Kalimok Field Station, NE Bulgaria (41°00'N, 26°26'E) were compared with those of passing birds. During the springs of 1998-2004, 591 individuals of this species were mist-netted and handled according to the guidelines in the Manual of Field Methods (ESF project) and to the South-East European Bird Migration Network (SEEN) standards. As local birds we identified 60 individuals caught at least twice in different seasons. The means of the fat score and the body mass calculated for the local birds and for the passing Great Reed Warblers were similar. Also, the time of the local birds arrival and the passage of the species in the region studied coincided. Therefore, we suggest that the southern and northern populations have equal potentials for realization of their migratory state and birds from southern populations could spread in spring to the north, being an important interregional gene flow in Great Reed Warblers.