Melanism-related prospecting behaviour likely affects re-nesting and annual fecundity in a nocturnal predator

Recent biologging technology helps reveal hidden life and breeding strategies of nocturnal animals. Combining animal trajectories and identified movement patterns with landscape features can reveal meaningful behaviours that directly influence fitness, such as prospecting for nesting sites. Consequently, defining the proximate mechanisms and adaptive value of the identified behaviours is of paramount importance. Breeding female barn owls ( Tyto alba ), a colour-polymorphic species, recurrently visit other nest boxes at night. We described and quantified this behaviour for the first time. Furthermore, we linked this behaviour with possible proximate drivers (partner-, individual- and brood-related variables), and fitness consequences, such as annual fecundity and productivity. We GPS-equipped 178 breeding pairs of barn owls from 2016 to 2020 in western Switzerland during the chick rearing phase. We observed that 65% of 171 breeding females tracked were (re)visiting other nest boxes while still carrying out their first brood. We characterized the behaviour calculating how many visits were completed, how much time was spent on visiting, and how many different nest boxes were visited. We modelled these parameters as a function of partner characteristics and movement behaviour. We also included the behavioural and morphological characteristics of the tracked female individual, and the ongoing clutch progress (brood size, chicks’ growth rate and deaths, etc.) during the tracking period. We then checked if visited nests were known from previous breeding attempts and if they were successful or at least used in current and previous years by any owl. We found that feather eumelanism predicted the emergence of prospecting behaviour (less melanic females are usually prospecting), while increasing male parental investment increased female exploratory efforts. Ultimately, females would revisit a nest more often if they had used it in the past. Also, the exploratory females were more likely to lay a second clutch afterwards and to have higher annual fecundity. Despite these apparent immediate benefits, they did not fledge more chicks than non-prospecting females. Although some aspects of this behaviour need further research, we highlight how biologging associated with field monitoring can be fundamental to understand individual behavioural plasticity and how this likely influences their reproductive fitness. ### Competing Interest Statement The authors have declared no competing interest.