SEEN - SE European Bird Migration Network
Delineating the migratory divide in the Reed Warbler (Acrocephalus scirpaceus) using stable isotopes and ringing recoveries - a progress report

Petr Procházka1, Keith Hobson, Zsolt Karcza, Jaroslav Cepák,
Wolfgang Fiedler, Rob Thomas, Sergio Scebba, Jiří Reif, Josef Beier, Mihaela Ilieva, Ričardas Patapavičius

1 Institute of Vertebrate Biology, Brno Czech Republic

Migration and wintering represent limiting periods in the life cycle of migratory birds, since adverse conditions en route and in the winter quarters can negatively influence breeding success and population sizes. Identification of wintering grounds of particular populations of migratory birds is thus essential for their conservation on a global scale. More than 100 years of bird ringing yielded significant insights into avian migration patterns. However, ringing activity and reporting probability vary significantly among regions. Therefore, alternative approaches such as the measurement of stable isotopes or trace elements in avian tissues are being adopted to overcome the possible bias in ring recovery data.
On their way to wintering grounds, migratory birds use population-specific directions that have a strong genetic component. About 18% of migratory passerines leave their European breeding grounds for their winter quarters in two different directions. Some populations of these species use the SW flyway via the Iberian Peninsula, whereas other head initially SE. A region, which more or less separates populations with such distinct directional preferences, is called migratory divide. A migratory divide represents thus a zone of contact between two parapatric populations migrating in two disparate directions. The aim of this presentation is a summary of recent results of an ongoing project aimed at delineating the presumed migratory divide in the Reed Warbler (Acrocephalus scirpaceus) in central Europe using stable isotopes and ringing recoveries.
Available ringing recoveries suggest that the majority of European Reed Warblers migrate SW, however, those breeding in the Pannonian plain (Carpathian basin) initially head towards SE. SW-migrating birds enter Africa mainly via Morocco and winter in western Africa from Senegal as far as western Nigeria. SE-migrating Reed Warblers enter Africa via Egypt and sub-Saharan recoveries point to the Lake Chad basin (S. Chad, E. Nigeria, N. Cameroon). The scarcity of encounters of ringed birds in southern direction suggests that the central Mediterranean route is used very rarely, thus supporting the existence of a migratory divide in this species.
Analysis of carbon and nitrogen stable isotopes from feathers grown in Africa and sampled at eight breeding sites across the migratory divide showed a strong overlap in the values of both δ15N and δ13C, which may suggest (1) a wide range of habitats used as moulting/wintering grounds, and/or (2) low migratory connectivity. Indeed, Reed Warblers have been repeatedly reported from both wet and dry habitats in Africa. Such ecological versatility of the species during winter makes use of the two stable isotopes for distinguishing disparately migrating populations difficult. A future study will focus on young birds (feathers of which grew at their natal sites) migrating through the western and eastern Mediterranean to infer the origin of birds at stopover sites and possibly on the use of deuterium isotope measurements.
The study has been supported by GA AV (grant no. B600930508).

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