Differences in Vocal Display During Territorial Encounters between Male Grasshopper Warblers Locustella naevia and River Warblers L. fluviatilis

Competition for territories and females inevitably places males of the same species into conflict. We investigated male-male interactions within Grasshopper Warbler, Locustella naevia (later GW) and River Warbler, Locustella fluviatilis (later RW) using acoustic playback experiments. Our aim was to clarify the vocal traits employed during intraspecific encounters and to compare the territorial vocal performance between two closely related species. In total, eight GW and ten RW males were tested in May and June 2019 along the Morava River, in western Slovakia. The GW males used two call types and reduced the song duration in response to conspecific song stimuli. The RW males used four call types, reduced and accelerated songs. In each species, we found a new type of call used in territorial interaction that had never been described and visualized before. The vocal response in territorial interactions was more diverse in RW males than in GW males. This finding is consistent with the higher complexity of RW spontaneous vocal performance within the population as well as on a broader geographic scale.