Birds can influence the environmental conditions that eggs and offspring experience by adjusting the composition of their nests. They often use feathers to build nests, presumably due to their insulating properties. The amount of feathers in nests is often associated with increased nestling survival and body condition. However, it is unclear whether these putative beneficial effects of adding feathers to nests are relevant in a wide range of environmental conditions or, instead, depend on the environment. Here, we combine data on weather conditions and feathers in nests (i.e. nest composition) to investigate their relative contribution to reproductive success of the Eurasian tree sparrow (Passer montanus). Specifically, we investigate whether the effect of weather conditions on breeding success is modulated by the amount of feathers added to the nest. We found a strong negative effect of rainfall on the number of nestlings that successfully fledged, but this negative effect was not mitigated by the amount of feathers in nests. We also found that the amount of feathers in nests varied across the breeding season. Nests contained more feathers early in the breeding season when temperatures were lower. Our results suggest that, despite considerable variation in nest composition, the use of feathers in nests is not beneficial to buffer negative rainfall effects on the reproductive success of tree sparrows.