201 Folia Zool. – 57(3): 201–211 (2008) When an alien sings at a rival’s post: a passerine excited by conspecific stimulus may show aggressive behaviour towards heterospecific individuals Dedicated to Professor Karel Hudec in honour of his 80th birthday Tereza PeTrusková 1 *, Adam PeTrusek 1 , václav PA vel 2 and roman Fuchs 3 1 Charles University in Prague, Faculty of Science, Department of Ecology, Vininá 7, CZ‑128 44 Prague 2, Czech Republic; * e‑mail: kumstatova@post.cz 2 Laboratory of Ornithology, Palacký University Olomouc, t. Svobody 26, CZ‑771 46 Olomouc, Czech Republic 3 Faculty of Science, University of South Bohemia, Branišovská 31, CZ‑370 05 České Budjovice, Czech Republic received 27 october 2007; Accepted 10 March 2008 A b s t r a c t . We tested the hypothesis that males of meadow pipits (Anthus pratensis) respond differentially to simulated territorial intrusions (song playback + bird model) by conspecific, related (tree pipit, Anthus trivialis), and unrelated individuals (willow warbler, Phylloscopus trochilus), and evaluated the effect of playback order. All tested males responded aggressively to the conspecific playback; reactions to heterospecific stimuli occurred only after previous excitation by the conspecific song and dummy, and were usually weaker. Apparently, although males were able to discriminate between playbacks, excitation by intrusion of a conspecific rival elicited an aggressive reaction to otherwise neutral stimuli. reactions to heterospecific playbacks closely following the conspecific one did not differ significantly between congeneric and unrelated species. The response to a subsequent heterospecific stimulus, however, was taxon- dependent: congeneric species elicited a significantly stronger reaction than an unrelated one. We presume that this was due to the morphological similarity between pipits, and that the more intensive reaction could have been caused by stronger visual stimulus from the respective model. We discuss the potential benefits as well as limitations of using models (dummies) in playback experiments, and suggest that overexcitation by previous stimuli should be considered during experimental setup and data analysis in studies focusing both on interspecific aggressiveness and on neighbour-stranger interactions. Key words: meadow pipit, Anthus pratensis, interspecific territoriality, playback experiments, misdirected aggressiveness Introduction correct species discrimination is crucial for bird species coexisting at the same localities. Identification of resident species is one of the mechanisms influencing habitat selection of migrants (M ö n k k ö n e n et al. 1996), and the ability to recognise conspecific and heterospecific competitors plays an important role in the competition for ecological resources (e.g., c a t c h p o l e 1978, G i l 1997, M a t y j a s i a k 2005, Sedláek et al. 2006). Discrimination between conspecifics and individuals of similar species is also crucial for maintaining pre-copulation reproductive barriers in closely related co-occurring taxa (e.g., B a k e r 1991, Q v a r n s t r ö m et al. 2006). Accurate species recognition in hybridising species is advantageous not only for mate recognition: an aggressive response of territorial * corresponding author