"Why do individuals vary in their cognitive abilities? This proposal takes the disciplines of cognition and evolutionary biology into a natural setting to answer this question by investigating a variety of proximate causes and population-level consequences of individual variation in cognitive ability. It represents the first large-scale integrative study of cognitive ability on any wild population. State of the art observational (remote sensing and automated self-administration trials of learning in the wild), chemical (stable isotope analysis of diet), physiological (stress, energetics, immunocompetence), molecular (DNA fingerprinting and metabarcoding) and analytical (reaction norm, quantitative genetic) techniques will be used. The chosen study system, the great tit Parus major, is one of the most widely used in Europe, but uniquely here will consist of 12 subpopulations across deciduous and conifer woodland fragments. The proposal’s broad scope is captured in three objectives: 1) To characterise proximate causes of variation in cognitive (associative/reversal learning; problem solving; brain size) and other traits (the reactive-proactive personality axis; bill morphology), all of which can influence similar ecologically important behaviour. Quantitative genetic, social, parasite-mediated, and physiological causes will be explored. 2) To examine links between these traits, and key behaviours and trade-offs, e.g., space use, niche specialization, predation, parental care and promiscuity; and 3) To examine the consequences of this variation for life histories and fitness. The research team consists of the PI, five early career biologists, and three PhD students, and will collaborate with eight researchers from Europe and further afield. The project will reveal ground-breaking insight into why individuals vary in their cognitive ability. It aims to impact a wide scientific community, to raise public interest in science, and to inform EU biodiversity policy."