Here, we present an analysis based on a 40-year dataset from a nest-box population of the collared flycatcher (Ficedula albicollis). Our objective is to examine the impact of weather conditions during the incubation and nestling period on key indicators of individual fitness, including offspring production and local annual recruitment rate. Our findings provide compelling evidence that climatic conditions experienced during both incubation and nestling periods significantly impact the number of fledglings and recruits. Specifically, we observed that higher precipitation during the nestling period negatively affects the number of fledglings and increases brood failure. Interestingly, higher precipitation during the incubation period is linked to increased recruitment numbers. Moreover, we found that warmer weather during both the incubation and nestling periods decreases brood failure, and more importantly, higher temperatures during the nestling period are positively associated with the number of recruits. These results underscore the complex interplay between weather patterns and avian reproductive strategies, highlighting the importance of long-term ecological studies in understanding the impacts of climate change on bird populations. By addressing the variability of climatic influences across different life stages, future research can help develop more comprehensive models for predicting the resilience of avian species in the face of ongoing climate challenges.

In the studied nest box population of the collared flycatcher inhabitant Swedish island of Gotland, breeding birds were monitored over the whole season from 1980 to 2020 to gather data including laying date, clutch size, and the number of fledglings and recruits. Females were trapped at the nest during incubation, and males were caught while feeding nestlings (May-June). After catching, each bird was banded with a metal band and aged as 1 year old or older. All breeding attempts were carefully monitored until the chicks fledged. On day 12 after hatching, nestlings were measured and banded with a metal band. In the study, we used the longitudinal dataset, containing all the records on annual female reproductive performance (i.e., the number of fledglings reared to independence and the number of recruits from each breeding attempt). However, for the recruitment rate, we utilized data only from the years 1980 to 2016. We excluded from the dataset all breeding events that were involved in any experimental manipulations. 

Daily temperature records and daily sums of precipitation were obtained from the meteorological station at Hoburgen (56.92 °N, 18.15 °E; approximately 10 km from the main study areas). The data were accessed via the Swedish Meteorological and Hydrological Institute (SMHI) open data portal. We calculated the average daily temperature and daily sums of precipitation during the incubation and nestling period. For the incubation period, we calculated the average daily temperature and the daily sums of precipitation over 13 days, starting from the day the last egg was laid. We chose the 13 days because previous work by Husby et al. (2012) showed that the mean incubation duration in this collared flycatcher population was 12.5 days. We assessed these parameters over 15 days for the nestling period, beginning on the final day of the determined incubation period. This duration was chosen based on our observations of collared flycatchers on Gotland Island, where fledglings typically leave the nest after an average of 16.2 days, with a range of 14 to 19 days (J. Sudyka, unpublished data).