Indirect fitness benefits of cognitive investment: Head size predicts adaptive nest-site selection in Barn Swallows (Hirundo rustica)

Cognitive traits such as brain size are hypothesized to influence avian reproductive strategies, yet the ecological mechanisms linking neural investment to fitness under natural conditions remain underexplored. According to the cognitive buffer hypothesis, larger brains should facilitate adaptive decision-making, potentially translating into indirect fitness benefits. In this study, we investigated the relationships among head size (a validated proxy for brain size), structural body size, nest-site selection, and reproductive success in Barn Swallows (Hirundo rustica) breeding in southern China. We found that head size was independent of structural body size metrics (e.g., tarsus length), confirming that it represents a distinct trait related to neural investment rather than allometric scaling. Although males possessed slightly larger heads than females, head size in both sexes was a significant predictor of fine-scale nest-site selection. Larger-headed individuals consistently selected nest sites located closer to overhead structures and opposing shelters. These specific architectural choices were, in turn, strong predictors of reproductive output: nests with greater enclosure produced significantly more fledglings, likely due to reduced predation risk and improved microclimatic buffering against extreme weather. Crucially, head size itself did not directly predict fledging success, suggesting that the fitness advantages of larger brains are realized indirectly through superior habitat selection decisions. Our findings provide empirical support for an indirect pathway between cognitive capacity and fitness, suggesting that individuals with larger cognitive proxies achieve higher reproductive success not by increasing fecundity per se, but by securing safer, buffered microhabitats for their offspring.