Nuthatch (Sitta europaea) responses towards simulated territorial intrusion (STI) vary in strength during the non-breeding season - a matter of temperature and photoperiod?

Many bird species defend their territories against conspecifics during the breeding season. Although some temperate species have been observed to defend territories outside of the breeding season, such cases are rarely noted in the literature, and it remains largely unknown which environmental factors influence such behaviour. In this study, we investigated non-breeding territorial behaviour in the Eurasian nuthatch (Sitta europaea). Territorial responses were elicited with a simulated territory intrusion (playbacks of calls combined with a wooden model of nuthatch) in autumn, winter and spring. Territorial responses did not differ between autumn and winter but differed between the autumn/winter and spring; they declined until about mid-December (winter) and increased after that into the spring. We found an increase in territorial responses with increasing photoperiod (day length) in autumn and with decreasing ambient temperature during spring. Our study provides novel evidence about non-breeding territorial behaviour in temperate birds and indicates that this behaviour is influenced by environmental factors.