Die Arealexpansion des Orpheusspötters

The reasons for moving range edges are manifold and their understanding is a central part of biogeographical research. Range shifts have been observed for many bird species. A clear distinction between biotic or abiotic factors is, however, difficult and mostly species-specific. The Melodious Warbler Hippolais polyglotta is a migratory passerine that breeds in the Western Palearctic. It has been expanding its range across Central Europe for more than 80 years. At the same time as this expansion, occurring in the same area is an ongoing range regression of its sister species, the Icterine Warbler H. icterina. Both species form a contact zone that moves alongside the shifting range edges of both species. Here we present a synthesis of previous research to understand the reasons
behind the shifting range edges in this system. Contrary to previous assumptions, climate change seems to not be directly responsible for the moving contact zone, while the competitive exclusion between the species is the reason for the formation of this range edge where both species meet. Also, an increased individual aggression potential could not be observed in the expansive Melodious
Warbler. The generally high mobility of both species further prevented negative genetic effects as a consequence from shifting range edges at the individual to population level. Clines in genetic diversity could neither be observed in expanding Melodious Warblers nor in receding Icterine Warblers. However, due to the changing local breeding densities of both species across the moving
contact zone over time, hybridization rarely occurs and leads to asymmetric introgression from the Icterine Warbler to the Melodious Warbler. The rare appearance of hybrids with apparently high genetic effects for the Melodious Warbler is a topic for future research.