In birds, females are often aggressive against conspecific females during the breeding. There are many explanations for this intrasexual aggression by females: e.g., nest site or food resource defence, prevention of intraspecific brood parasitism or infanticide, and monopolization of paternal care. A pair might also benefit by defending an alternative nest site in addition to the current nest site. Here, I study by experimentation whether pied flycatcher (Ficedula hypoleuca) females defend an extra nest box as an alternative nesting site. Further, I examine whether females behave differently when their mate is present vs not present. I measured the reactions by resident females to a caged female intruder when provided with an extra nest box. I did not find any evidence that females defended an extra nest box. However, females spent less time near the caged intruder when their mate was present and instead seemed to guard him, most likely to secure his contribution to feeding the young. Females were also aggressive when their mate was absent and when there was no alternative nest site suggesting the importance of defending the current nest site per se.