SEEN - SE European Bird Migration Network
Directional preferences of the Sedge Warbler (Acrocephalus schoenobaenus L.) during autumn migration in western Ukraine
Oksana Zakala
Western Ukrainian Ornithological Station and Zoological museum of Ivan Franko National University of Lviv, Ukraine

The study of the spatial orientation of the Sedge Warbler in the western Ukraine was carried out in the "Cholginski" ornithological reserve during last nine years (from 1996 till 2004). In total 414 orientation experiments were done, in 379 of them directional preference was significantly different from random (see Table 1).

Table 1. Terms and number of orientation experiments done in the "Cholginski" ornithological reserve.
Year Date ** N Number
of experiments
1996 19.08-05.09 42 1 43
1997 03.08-27.08 24 1 25
1998 02.08-20.08 29 3 32
1999 22.07-29.08 36 1 37
2000 01.08-15.08 33 7 40
2001 09.08-27.08 56 6 62
2002 04.08-30.08 35 11 46
2003 06.08-28.08 63 4 67
2004 03.08-26.08 61 1 62
    379 35 414

One of the goals of this study was to find out if bird activity in the cage depends on physiological parameters (fat content, weight) or environmental conditions (time, sky conditions, wind direction). However, I found no relationship between the bird activity level and the parameters and factors listed above.
Sedge Warblers choose three directions during the autumn migration in the western Ukraine, i.e.: SE, SSW, WSW. SE direction is the most prominent, while the WSW direction is less distinct.
Since all orientation experiments were conducted each year in the same period (except 1999, when experiments were conducted also in July), I could analyze all data together, or compare annual results with each other.
All three directions mentioned above were clearly pronounced in most years (1996-97, 99, 2003-2004). Only two (in 1998, 2001-2002) or even just one (2000) direction dominated in the other years. SE direction predominated in 2000 and 2002, SSW – in 1997-1999 and in 2003, WSW – only in 2001.
Presence of some minor directions observed could be explained by the errors resulting from the stress of a bird, mistakes and attempts to escape from the cage.
Directional preferences in 1999 were not clear. I think that a cause for such results could be terms of the study (end of July – end of August), as well as age of tested birds, i.e. immature birds that could be during post-fledging dispersal.
Similar orientation experiments were held in Poland in "Jezioro Druzno" reserve. SSW direction was also found in Poland (45%), just like in Ukraine. However, on the territory of Ukraine the SE direction is a dominating one, probably because larger part of birds that fly through the territory of the western Ukraine, migrates along the south-east direction. It is known that the Sedge Warbler from the south Scandinavia and western Europe winter in western Africa, whereas birds from Finland and eastern Europe winter in central and eastern Africa.
In 77,3% of tests out of 379 analysed, birds chose one direction, while in 22,7% - two or more. The latter group consists of interpopulationary hybrid individuals (in the sense of migrational populations), and can have two or more migratory directions, coded in their genome. It was also found that most young birds chose just one direction, while most adults – two or more.
I tried to verify the hypothesis about the occurrence of more than one population of the Sedge Warbler migrating through the territory of Western Ukraine.
I searched for morphometric differences between birds divided into groups according to their orientation preferences. I compared bill, tarsus, tail and wing length, with a special attention to the wing length. There was a significant difference in bill length (0,96) between SSW and WSW and between SE and WSW group of birds, significant difference in wing length (0,96) between SE and WSW groups of birds, and close to significant difference in wing length (0,94) between SE and SSW groups of birds. An analysis of young and adult individuals revealed no additional differences.

Data on ringed and recovered birds support the existence of south-western migration directions, while there is no information at all that can support the migration of the Sedge Warbler along the south-east flyway. However number of recovered birds depends not only on a number of ringed individuals, but also on human population density and cultural traditions in regions along the migratory route. Thus, the number of recovered Sedge Warblers in the south and south-eastern regions of Europe is still small.
I also found some differences between the main migration directions of adult and young birds. Most adult birds preferred WSW direction, while young birds – SE and SSW directions. The standard choice of  three directions may be genetically determined. Moreover, we think that young birds prefer genetically determined migratory direction while adults choose the route taking into consideration also previous migratory experience.
The presence of three distinct directions also may be caused by the presence of more than one different Sedge Warbler populations, which fly through the territory of western Ukraine.
Created by Pronetix 2006