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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 experience, age, lifespan and resource availability. Variation in these energetic investments can be described as different demographic tactics which are expected to optimize the fitness of local populations. These ideas are largely supported by both empirical and model-based studies but research identifying specific strategies and their corresponding environmental drivers within wild populations remains rare. Using 12 years of data, we investigated reproductive investment strategies in a relatively short-lived resident songbird, the mountain chickadee (Poecile gambeli), at two elevations that differ in environmental harshness in the North American Sierra Nevada mountains. Challenging winter environments at high elevations impose strong selection pressure on survival-related traits (e.g. specialized spatial cognition associated with food caching) and significantly shorten the length of the reproductive window. Here, we show that chickadees at a higher elevation lay smaller clutches (ca0.41 fewer eggs) and produce fewer (ca0.25 fewer nestlings) but larger offspring (ca0.4 g heavier) compared to lower elevation residents. Due to the harsher and less predictable environmental conditions at higher elevations, this investment strategy in this resident species 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 along strong environmental gradients. 

Cognitive abilities are hypothesized to affect survival and life span in nonhuman animals. However, most tests of this hypothesis have relied on interspecific comparisons of indirect measures of cognitive ability, such as brain size. We present direct evidence that individual variation in cognitive abilities is associated with differences in life span in a wild food caching bird. We measured the spatial cognitive abilities and tracked the life span of 227 mountain chickadees (Poecile gambeli) in their natural environment and found that individuals with better spatial learning and memory abilities involved in food caching lived longer. These results confirm that enhanced cognitive abilities can be associated with longer life in wild animals and that selection on cognitive abilities can lead to increased life span. 

Wild populations appear to synchronize their reproductive phenology based on numerous environmental and ecological factors; yet, there is still individual variation in the timing of reproduction within populations and such variation may be associated with fitness consequences. For example, many studies have documented a seasonal decline in reproductive fitness, but breeding timing may have varying consequences across different environments. Using 11 years of data, we investigated the relationship between relative breeding timing and reproductive success in resident mountain chickadees (Poecile gambeli) across two elevational bands in the Sierra Nevada mountains, USA. Chickadees that synchronized breeding with the majority of the population (‘peak’ of breeding) did not have the highest breeding success. Instead, birds that bred early performed best at high elevation, while at low elevation early and peak nests performed similarly. At both elevations, late nests consistently performed the worst. Overall, breeding success decreased with increasing relative timing at both high and low elevations, but the relationship between breeding success and timing differed among years. Our results suggest that in mountain chickadees, earlier breeding is associated with higher reproductive success, especially at high elevations, while late breeding is consistently associated with lower reproductive success at both elevations. 

While researchers have investigated mating decisions for decades, gaps remain in our understanding of how behaviour influences social mate choice. We compared spatial cognitive performance and food caching propensity within social pairs of mountain chickadees inhabiting differentially harsh winter climates to understand how these measures contribute to social mate choice. Chickadees rely on specialized spatial cognitive abilities to recover food stores and survive harsh winters, and females can discriminate among males with varying spatial cognition. Because spatial cognition and caching propensity are critical for survival and likely heritable, pairing with a mate with such enhanced traits may provide indirect benefits to offspring. Comparing the behaviour of social mates, we found that spatial cognitive performance approached a significant correlation within pairs at low, but not at high elevation. We found no correlation within pairs in spatial reversal cognitive performance at either elevation; however, females at high elevation tended to perform better than their social mates. Finally, we found that caching propensity correlated within pairs at low, while males cached significantly more food than their social mates at high elevations. These results suggest that cognition and caching propensity may influence social mating decisions, but only in certain environments and for some aspects of cognition.