Smart and safe? Antipredator behavior and breeding success are related to head size in a wild bird

Increased brain size has been associated with greater sensitivity to environmental context, but this flexibility is potentially costly as sampling the environment is time and energy consuming and may even increase the risk of predation. However, these potential trade-offs remain virtually unexplored in natural populations. We hypothesized that large brain size is 1) beneficial under challenging conditions and allows better matching of antipredator responses to the actual threat by predators and 2) associated with thorough risk assessment, which can be costly under benign conditions. To test these hypotheses, we examined the relationship between relative head volume, reproductive decisions, and fitness components in female common eiders (Somateria mollissima) under variable predation risk and breeding phenologies. This species is ideal for this purpose because of highly variable predation pressure and a distinct seasonal decline in reproductive success. The results were consistent with our hypotheses. First, females with depredated nests had smaller heads than expected by chance when predation rate (killed females/nest) was highest ("challenging conditions"). Second, large-headed females, but not small-headed ones, took a shorter time to form antipredator brood-rearing coalitions in more dangerous years. Third, large-headed females had a later onset of breeding, and their nests were more likely to be depredated when annual median nesting was earliest and predation pressure on females was low ("benign conditions"). Thus, predation risk and annual phenology may exert temporally fluctuating selection on relative head size, maintaining intraspecific variation in cognitive ability.