Interspecific territoriality can frequently be observed in the animal kingdom, including birds. One important factor driving the degree of interspecific territoriality is competition for resources. Other influential factors are, for instance, population density, phylogenetic relatedness, fighting asymmetries, and shared predation. Although the degree of competition is usually a composite of several factors potentially influencing competition, it is possible to determine the degree of competition without knowing all influential factors and their importance. In line with this, we used the strength of behavioural interactions between species under standardized experimental conditions. In particular, we investigated the degree of interspecific competition using the territorial song of male Eurasian Blackcaps (Sylvia atricapilla) as a low-risk response variable and the crossing flights and nearest approach distance to an intruder as high-risk defence behaviours. To simulate an interspecific intruder, we used male song playbacks of phylogenetic-related and more or less coexisting species. In line with our predictions, we found territorial defence to be most intense against conspecifics (positive control) and virtually non-existent against the allopatric Subalpine Warblers (S. cantillans) (negative control). In terms of the defence reaction to sympatric interspecific intruders, the response seemed strongest in relation to the Garden Warbler (S. borin) and weakest in relation to the Barred Warbler (S. nisoria). Our results further show that the vegetation characteristics of a territory influence behavioural responses to intruders, but only in relation to low-risk defence.