Migratory state: body mass and fat level of some passerine long-distance migrants during autumn migration in north-eastern Bulgaria
Body condition of 3224 migrating birds of Great Reed Warbler (Acrocephalus arundinaceus), Sedge Warbler (A. schoenobaenus), Willow Warbler (Phylloscopus trochilus), Red-backed Shrike (Lanius collurio) and Whitethroat (Sylvia communis) was studied at Kalimok Station, NE Bulgaria (41°00'N 26°26'E) in the autumns of 1997-2003. The mean values of the fat level and the body mass were calculated for each species and year and were compared with similar data of various parts of the Palaearctic-African Migration System. Distinct variations in the mean values of studied parameters in different years were recorded. The dynamics of the accumulated subcutaneous fat and the body mass during the autumn varied in different species and either increase or decrease of the values during a certain season were registered. Age-determined differences in the studied characteristics were found. The timing of the passage of the first-year and adult birds can influence the dynamics of the mean fat level and the mean body mass. The lack of selective environment in NE Balkan Peninsula allows the passage of conspecific birds in various migratory states. The realisation of their migration adaptations highly depends on environmental conditions during the period of passage.
Directional behaviour of the Sedge Warbler (Acrocephalus schoenobaenus) studied in two types of orientation cages during autumn migration - a case study
Migratory directions of Sedge Warblers tested in the Emlen funnel and Busse's flat cage during autumn migration in the Balkan region were studied. Some methodological aspects of orientation data analysis were discussed as well. According to orientation data at least two populations of the Sedge Warbler migrate in NE Bulgaria: one in SW direction via Greece, crossing the Mediterranean Sea to Libya, and then Central Africa and second one through Turkey, (Cyprus?) and the Middle East to the eastern parts of winter quarters (SE direction). This pattern is clear when a calculation method that assumes reverse and axial behaviour of birds (i.e. reversing vectors from northern sectors) is applied. Nature of "reverse migration" is still not well recognised and seems to be very interesting for further studies.