These warblers are mostly found in open woodland, reedbeds or tall grass. They occur mostly in southern to western Eurasia and surroundings, but also ranging far into the Pacific, with some species in Africa. Three genera show hybridization, namely Acrocephalus, Iduna, and Hippolais.
Acrocephalus
Hybrids between species of the genus Acrocephalus have been documented by studying mixed breeding pairs, for example Great Reed Warbler (A. arundinaceus) x Eurasian Reed Warbler (A. scirpaceus) in Russia (Pukas, 1989) or Blyth’s Reed Warbler (A. dumetorum) x Marsh Warbler (A. palustris) in the Netherlands (Poot, Engelen & Van Der Winden, 1999). Also, mixed songs hint towards hybridization (Lemaire, 1977). However, molecular techniques allow to identify putative hybrids. The following studies applied microsatellites (sometimes in combination with mtDNA) to confirm the hybrid origin of certain birds:
- Great Reed Warbler (A. arundinaceus) x Eurasian Reed Warbler (A. scirpaceus) in Germany (Beier, Leisler & Wink, 1997), Belgium (Hansson, Roggeman & De Smet, 2004) and Romania (Ion et al., 2012)
- Marsh Warbler (A. palustris) x Sedge Warbler (A. schoenobaenus) in Norway (Lifjeld et al., 2010)
- Great Reed Warbler (A. arundinaceus) x Clamorous Reed Warbler (A. stentoreus) in Kazachstan (Hansson, Gavrilov & Gavrilov, 2003)
The latter hybrids in Kazakhstan have been studied in greater detail. Hybridization does not seem to result in backcrossing or introgression (Hansson et al., 2012). Isotope analysis revealed that Great Reed Warblers migrate to sub-Saharan Africa, while Clamorous Reed Warblers prefer India. Hybrids tend to follow Great Reed Warblers (Yohannes et al., 2011).
Great Reed Warbler and Clamorous Reed Warbler
Hippolais
The Icterine Warbler (H. icterina) and the Melodious Warbler (H. polyglotta) can be easily discriminated based on wing characteristics and song. However, the Melodious Warbler is replacing the Icterine Warbler in Western Europe. In a moving hybrid zone, wing morphology (Faivre et al., 1999) and song (Secondi et al., 2003; Secondi, Faivre & Kreutzer, 1999) seem to be converging. Song convergence can be partly explained by genetic effects due to hybridization (Secondi et al., 2011). Furthermore, introgression appears to be limited to nuclear loci (Secondi, Faivre & Bensch, 2006). Species Distribution Modelling indicated that the movement of the hybrid zone is not driven by climate change, but likely by interspecific interactions (Engler et al., 2013).
This hybrid zone has also been used to study the dynamics of blood parasite infections. In general, parasite distributions fitted nicely with the geographical range of their host species, which means that the hybrid zone can act as a barrier to parasite expansion. However, some parasites spread from the expanding host (Melodious Warbler) into the receding one (Icterine Warbler), which may lead to parasite-mediated competition (Reullier et al., 2006).
Icterine Warbler and Melodious Warbler
References
Beier, J., Leisler, B. & Wink, M. (1997). A Great Reed x Reed Warbler (Acrocephalus arundinaceus x A-scirpaceus) hybrid and its parentage. Journal Fur Ornithologie 138, 51-60.
Engler, J. O., Rodder, D., Elle, O., Hochkirch, A. & Secondi, J. (2013). Species distribution models contribute to determine the effect of climate and interspecific interactions in moving hybrid zones. Journal of Evolutionary Biology 26, 2487-2496.
Faivre, B., Secondi, J., Ferry, C., Chastragnat, L. & Cezilly, F. (1999). Morphological variation and the recent evolution of wing length in the Icterine Warbler: a case of unidirectional introgression? Journal of Avian Biology 30, 152-158.
Hansson, B., Gavrilov, E. & Gavrilov, A. (2003). Hybridisation between great reed warblers Acrocephalus arundinaceus and clamorous reed warblers A. stentoreus: morphological and molecular evidence. Avian Science 3, 145-151.
Hansson, B., Roggeman, W. & De Smet, G. (2004). Molecular evidence of a reed warbler x great reed warbler hybrid (Acrocephalus scirpaceus x A-arundinaceus) in Belgium. Journal of Ornithology 145, 159-160.
Hansson, B., Tarka, M., Dawson, D. A. & Horsburgh, G. J. (2012). Hybridization but No Evidence for Backcrossing and Introgression in a Sympatric Population of Great Reed Warblers and Clamorous Reed Warblers. Plos One 7.
Ion, C., Bolboaca, L., Ciorpac, M., Stefan, A. & Gorgan, D. L. (2012). A Great Reed Warbler x Reed Warbler hybrid (Acrocephalus arundinaceus x Acrocephalus scirpaceus) in northeastern Romania. Journal of Ornithology 153, 975-978.
Lemaire, F. (1977). Mixed Song, Interspecific Competition and Hybridization in Reed and Marsh Warblers (Acrocephalus-Scirpaceus and Palustris). Behaviour 63, 215-240.
Lifjeld, J. T., Marthinsen, G., Myklebust, M., Dawson, D. A. & Johnsen, A. (2010). A wild Marsh Warbler x Sedge Warbler hybrid (Acrocephalus palustris x A. schoenobaenus) in Norway documented with molecular markers. Journal of Ornithology 151, 513-517.
Poot, M., Engelen, F. & Van Der Winden, J. (1999). Een gemengd broedgeval van Struikrietzanger Acrocephalus dumetorum en Bosrietzanger A. palustris bij Utrecht in voorjaar 1998. Limosa 72, 151-151.
Pukas, A. (1989). The cases of mixed pairs of reed (Acrocephalus scirpaceus) and marsh (A. palustris) warblers (Passeriformes, Sylviidae). Zool. Shurn, 68, 153-157.
Reullier, J., Perez-Tris, J., Bensch, S. & Secondi, J. (2006). Diversity, distribution and exchange of blood parasites meeting at an avian moving contact zone. Molecular Ecology 15, 753-763.
Secondi, J., Bordas, P., Hipsley, C. A. & Bensch, S. (2011). Bilateral Song Convergence in a Passerine Hybrid Zone: Genetics Contribute in One Species Only. Evolutionary Biology 38, 441-452.
Secondi, J., Bretagnolle, V., Compagnon, C. & Faivre, B. (2003). Species-specific song convergence in a moving hybrid zone between two passerines. Biological Journal of the Linnean Society 80, 507-517.
Secondi, J., Faivre, B. & Bensch, S. (2006). Spreading introgression in the wake of a moving contact zone. Molecular Ecology 15, 2463-2475.
Secondi, J., Faivre, B. & Kreutzer, M. (1999). Maintenance of male reaction to the congeneric song in the Hippolais warbler hybrid zone. Behavioural Processes 46, 151-158.
Yohannes, E., Lee, R. W., Jochimsen, M. C. & Hansson, B. (2011). Stable isotope ratios in winter-grown feathers of Great Reed Warblers Acrocephalus arundinaceus, Clamorous Reed Warblers A. stentoreus and their hybrids in a sympatric breeding population in Kazakhstan. Ibis 153, 502-508.
Avian Hybrids 