Weather conditions across reproductive stages affect brood size and nestling mass in a montane songbird

Reproductive success in wild animals can be associated with prevailing climatic conditions across the breeding period, and these patterns may be attributed to both direct (e.g., excessive heat, freezing) and indirect (e.g., mediated by parental behavior) effects of climate. Parents experiencing inclement weather may adjust their behavior at different stages of reproduction to strike a balance between self-maintenance and investment in offspring. Understanding how weather conditions affect reproductive success is increasingly valuable in the era of global climate change which is associated with both directional (warming) and nondirectional (increasing frequency of extremes) shifts in weather patterns. Here, we investigated how weather conditions and the presence and number of extreme climate events (5th and 95th percentile) during different stages of breeding affected reproductive success in Poecile gambeli (Mountain Chickadee). Based on 13 yr of data from 892 nests at 2 elevational sites in the Sierra Nevada, USA, we show that variation in both ambient temperature and precipitation was associated with differences in reproductive success, but associations were not uniform across elevations and depended on the reproductive stage. Lower temperatures, more precipitation, more extreme cold events and more extreme precipitation events during egg laying were associated with smaller brood sizes at both elevations. More precipitation and more extreme precipitation events during incubation were also associated with decreased brood sizes, but intermediate average temperatures during incubation were associated with the largest brood sizes at both elevations. Temperature during the nestling stage was associated with differences in nestling mass, but these relationships differed between high and low elevations. Our results show that weather conditions during reproduction can have important effects on reproductive success, but such effects vary depending on reproductive stage and across environments.