Territorial fidelity of willow warbler (Phylloscopus trochilus) males and success in competition over territories

Summary Territorial fidelity and competition between males was studied in a population of individually marked willow warblers (Phylloscopus trochilus). After spending the winter in Africa males arrive in the study area in late April or the beginning of May. They can be separated into two categories, old colour-banded males that return for a new breeding attempt, and birds that are new to the study area. These new birds could either be birds that had bred the previous year returning to a different site, or more likely 1-year-old birds returning for their first breeding attempt. Old males show high site fidelity and are successful in retaining their territories from the preceding year. The few old males that switched territories between years seemed to do so in response to reduced breeding success in the preceding year. However, in 13 out of 14 cases when a new resident was already established on the previous resident's territory, the old male was successful in evicting the new resident. In a removal experiment both old and new birds were removed from their territories and then released when a replacement male had been established. All old males successfully managed to reclaim their original territories after escalated contests. None of the new males were successful and no fights were observed.