Abrupt spring frost is associated with reduced egg investment but not hatching success in a montane Pied Flycatcher population

Climate change is altering not only average temperatures but also the frequency and intensity of climatic extremes, including late-spring frost events following unusually warm periods. These abrupt temperature fluctuations can disrupt trophic interactions by damaging newly emerged vegetation and reducing food availability for insectivorous birds during reproduction. However, their effects on maternal reproductive investment remain poorly understood. We used a natural experiment in a montane population of Pied Flycatchers ( Ficedula hypoleuca ) breeding in central Spain to evaluate whether an exceptional and pronounced spring frost in 2024 influenced reproductive decisions. We compared laying date, egg volume, and hatching success across three consecutive breeding seasons (2023–2025), combining reproductive monitoring with temperature records and satellite-derived vegetation activity (NDVI). Vegetation activity was lower during the frost year, coinciding with severe canopy defoliation following an abrupt temperature decline after an unusually warm early spring. Females laid significantly smaller eggs during the frost year than in the preceding and following breeding seasons, whereas laying date and hatching success showed comparatively limited interannual differences. These findings are consistent with the hypothesis that females reduced reproductive investment when energetically constrained while maintaining overall reproductive performance. Low within-female repeatability of egg volume suggests high phenotypic plasticity in response to environmental conditions. Our study suggests that short-lived climatic extremes may influence reproductive allocation even when downstream reproductive success remains relatively unaffected. These findings highlight the importance of considering extreme weather events, alongside long-term warming trends, when evaluating avian responses to climate change.