Environmental conditions but not nest composition affect reproductive success in an urban bird

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 miti...