The optimal compromise between decision speed and accuracy may depend on cognitive ability, associated with the degree of encephalization: larger brain size may select for accurate but slow decision-making, beneficial under challenging conditions but costly under benign ones. How this brain size-dependent selection pressure shapes avian breeding phenology and reproductive performance remains largely unexplored. We predicted that (1) large-brained individuals have a delayed breeding schedule due to thorough nest-site selection and/or prolonged resource acquisition, (2) good condition facilitates early breeding independent of relative brain size, and (3) large brain size accrues benefits mainly to individuals challenged by environmental or intrinsic constraints. To test these predictions, we examined how the relative head volume of female eiders (Somateria mollissima) of variable body condition correlated with their breeding schedule, hatching success and offspring quality. The results were consistent with our predictions. First, large head size was associated with a progressively later onset of breeding with increasing breeding dispersal distance. Second, increasing body condition advanced the timing of breeding, but this effect was significantly weaker in large-brained females. Third, larger head volume was associated with increased hatching success mainly among late breeders and those in poor body condition, and duckling body condition was positively related to maternal head volume, but only in poor-condition mothers. Our study is, to our knowledge, the first to demonstrate the presence of brain size-related differences in reproductive strategies within a single natural population.
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We thank Petteri Lehikoinen, Heikki Eriksson, James Montanari, Martin Seltmann, Sara Neggazi and Ben Steele for their assistance in the field. We thank Anders Møller for discussion and constructive comments, and two anonymous reviewers for their comments. Tvärminne Zoological Station provided excellent facilities. This study was funded by the Academy of Finland (grants no. 266208 to KJ and 128039 to MÖ), the Finnish Cultural Foundation (to KJ) and the Swedish Cultural Foundation in Finland (to MÖ).
Author contribution statement
KJ and MÖ conceived and designed the study and both authors collected the field data together. KJ analysed the data. KJ and MÖ wrote the manuscript.
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