**Abstract** The size and growth patterns of nestling birds are key determinants of their survival up to fledging and long-term fitness. However, because traits such as feathers, skeleton and body mass can follow different developmental trajectories, our understanding of the impact of adverse weather on development requires insights on trait-specific sensitive developmental windows. Using data of 2693 nestling Alpine swifts (*Tachymarptis melba*) measured over 25 different years, we show that the sensitive developmental windows for wing and sternum length corresponded to the periods of peak growth, with adverse weather conditions during these periods slowing down growth and reducing size at 50 days (i.e. close before fledging). Although nestling body mass at 50 days showed the greatest inter-individual variation, this was explained by weather in the 2 days before measurement rather than during peak growth. Interestingly, this relationship between weather and body mass followed a bell shape, with a sharp drop in body mass on cold days and a drop, albeit moderate, on hot days, the latter likely caused by heat stress. Nestlings experiencing adverse weather conditions during growth had lower survival rates up to fledging and fledged at later ages, presumably to compensate for slower wing growth. Overall, our results suggest that measures of feather growth and, to some extent, skeletal growth best capture the consequences of adverse weather conditions throughout the whole development of offspring, while body mass better reflects the short, instantaneous effects of weather conditions on their body reserves (i.e. energy depletion vs storage in unfavourable vs favourable conditions).