KROON G DE, M MOMMERS & H SLABBEKORN (2008) Vocal variation in the water Rail Rallus auaticus a comparison between two subspecies. LIMOSA 81 (3): 81-91.

Water Rails are often highly vocal and the European subspecies (Rallus aquaticus aquaticus) is famous for its advertisement call reminiscent of a squealing pig. These calls are likely to be important in defending territories and attracting mates in the dense vegetation of their marsh habitat. Species-specific acoustic features are critical for these functions, but little is known about acoustic variation within and between different subspecies of the Water Rail. There are four subspecies of which Rallus aquaticus aquaticus in Europe, northern Africa, and western Asia, and Rallus aquaticus indicus in eastern Asia are the most wide-spread (fig. 1). We have analysed a replicated set of sound recordings of both subspecies, obtained in the Netherlands and in Japan. Calls of different individuals revealed considerable variation, while repeated recordings of the same individual showed a highly stereotypic call structure. For aquaticus we recorded two call types: a relatively long and complex call (the pig squeal), and a short and simple call, both repeated in series. For indicus we only recorded calls of the complex type, which were clearly distinct fromthose of its European counterpart (non pig-like). All call types start with a low-pitched growl, soon followed by a coinciding, higher-pitched component, gradually and sometimes abruptly modulated in frequency, especially in the complex call (fig. 2). The complex call of indicus has a lower-pitched growl and a distinctly shorter duration. The simple call of aquaticus is shorter and discretely different in structure from the complex call of both subspecies (figs 3-5). Lack of response of aquaticus to complex calls of indicus in pilot playback experiments suggests biological significance for these differences. More research is needed on geographic variation in call structure across the species' Eurasian distribution (mainly the subspecies 'Korejewi' in Central Asia), and on genetic differences. Furthermore we need well-replicated playback experiments to assess whether acoustic differences between subspecies indeed reveal a lack of response to other subspecies' calls.

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