The two European species of treecreepers, the short-toed treecreeper Certhia brachydactyla and the common treecreeper C. familiaris, occupy different habitats, but are sympatric to a certain extent. The response to specific and heterospecific song was studied both in a sympatric and in an allopatric population of short-toed treecreeper in the Iberian Mountains (Burgos, Castilla-León, Spain). Short-toed treecreepers in sympatry showed a more aggressive response towards the song of the common treecreeper than those in allopatry. Although strict interspecific territoriality was not found, the experimental data suggest that the altitudinal distribution pattern of the two species may be a consequence of the increased aggression. This interspecific aggression may be advantageous for the short-toed treecreeper, as it may achieve a loose interspecific territoriality resulting in some spacing. However, a non-adaptive hypothesis for this behaviour cannot be rejected.