Morphological variation and the recent evolution of wing length in the Icterine Warbler:: a case of unidirectional introgression?

Icterine Warbler Hippolais icterina and Melodious Warbler H. polyglotta are closely related species with parapatric breeding ranges. Their breeding ranges overlap only in a narrow zone in western Europe, where the Icterine Warbler population is presently declining and the Melodious Warbler population expanding. In eastern Burgundy (France), both species have bred in sympatry for at least forty years, but the Icterine Warbler started to decline in the mid-seventies. The two species differ in wing length and wing formula. Morphological variations of the wing were compared for the Icterine Warbler population in eastern Burgundy between 1965/76 and 1985/96. In the second period, wing length and wing characteristics of the Icterine Warbler approached those of the Melodious Warbler. The number of recorded heterospecific pairs also differed between the two periods. No similar morphological change was observed for the Melodious Warbler between the two periods. We discuss the evidence for unidirectional generic introgression from the Melodious into the Icterine Warbler population as a likely explanation for this observed situation.