Interactions between sibling species of treecreepers Certhia familiaris and C. brachydactyla in the Pyrenees and the mistaken identity hypothesis

Abstract

ABSTRACT Capsule Aggressive interactions between two treecreeper species were probably the result from misidentification of heterospecifics rather than resource competition. Aims To test the alternative hypotheses that aggressive interactions between two sibling species, the Eurasian Treecreeper Certhia familiaris and the Short-toed Treecreeper Certhia brachydactyla, were the result of resource competition or misidentification of the species involved. Methods Interspecific interactions were investigated by direct observation and territory mapping of the two species during 12 years in a 25 ha plot in the Pyrenees. In addition, we performed playback experiments to test the behavioural response of both species to the heterospecific song in three sympatric areas of the Pyrenees, and in an adjacent area where only the Short-toed Treecreeper occurred. Results Territories largely overlapped between the two species. Subsequently, distance between the two species’ nearest nests was significantly shorter than distance between the same species’ nearest nests. Overall, the Short-toed Treecreeper was more aggressive towards the sibling species than the Eurasian Treecreeper. However, in contrast to what might be expected according to the resource-competition hypothesis, it was not significantly more aggressive in sympatry than in allopatry. Conclusion These results suggest that the aggressive behaviour of the Short-toed Treecreeper was mainly caused by the birds mistaking the identity of Eurasian Treecreepers, and had little impact on the cohabitation between the two species.