Supplementary material from "Small scale, elevation- and environmental-related differences in life history strategies in a temperate resident songbird"

Environmental drivers of within-population reproductive patterns are often hypothesized to lead to reproductive strategies tuned to local conditions. Organisms adjust energy allocation between survival and reproduction based on age, lifespan, and resource availability. Variation in energetic investment can be described as different demographic tactics which are expected to optimize fitness. These ideas are largely supported by empirical and model-based studies but identifying specific strategies and corresponding environmental drivers remains rare. Using 12 years of data, we investigated reproductive investment strategies in a short-lived resident songbird, the mountain chickadee, at two elevations that differ in environmental harshness in the Sierra Nevada mountains. Challenging winter environments at high elevations impose strong selection pressure on survival-related traits (e.g. specialized spatial cognition) and shorten the reproductive window. Here we show that chickadees at a higher elevation lay smaller clutches (ca. 0.41 fewer eggs) and produce fewer (ca. 0.25 less nestlings) but larger offspring (ca. 0.4 grams heavier) compared to lower elevation residents. Due to the harsher and less predictable conditions at higher elevations, this strategy likely leads to the production of offspring with greater chances of survival. Overall, our results show that within-species differences in life-history strategies may evolve over a small spatial scale.