Capsule: Using an experimental approach, this study disentangles effects of two important sources on the elicitation of aggressive interspecific behaviour. Aims: Although interspecific competition for nest sites is broadly assumed to be an example of interference, there are no experimental studies supporting this idea. Here the importance of competition for nest sites and food for the elicitation of aggressive behaviour of a secondary cavity-nesting bird are investigated. Methods: The Collared Flycatcher’s Ficedula albicollis responses to mounts of the Great Tit Parus major (nest site and food competitor), Chaffinch Fringilla coelebs (food competitor) and Dunnock Prunella modularis (control species) were tested. Trials were performed near flycatchers’ nests during nest building, incubation and care of nestlings. Results: The intensity of dive attacks and frequency of contact attacks showed that the aggression of Collared Flycatchers decreased in the direction: Great Tit > Chaffinch > Dunnock. The difference in aggressiveness was greater between Great Tit and Chaffinch than between Chaffinch and Dunnock. Aggression directed toward Great Tits increased from the nest building to the incubation stage and then decreased markedly in the nestling stage. Males were more aggressive than females. Conclusion: These results suggest that competition for nest sites, and to a lesser extent for food, may be of an interference nature and that the reproductive value hypothesis can only partly explain differences in the Collared Flycatcher’s defensive behaviour found between sexes and stages of the breeding cycle.