Figure 3 in Hybridisation or vocal mimicry? A case of a mixed singing River Warbler Locustella fluviatilis in eastern Poland

Figure 3. Sonogram image comparison. River Warbler typical song – single note longer, main frequency spectrum wider, harmonics visible as horizontal bars or waves, no double narrow sharp elements. Grasshopper Warbler typical song – single note shorter, main frequency spectrum narrower, no visible harmonics bars, double narrow sharp elements within each note. RW – River Warbler-type song of mixed singer; GW – Grasshopper Warbler-type song of mixed singer (recordings of the typical songs made by T. Ogrodowczyk, available in Kruszewicz, 2006).