Long-term studies of free-living vertebrate populations have proved a rich resource for understanding evolutionary and ecological processes, because individuals’ life histories can be measured by tracking them from birth/hatching through to death. In recent years the ‘animal model’ has been applied to pedigreed long-term study populations with great success, dramatically advancing our understanding of quantitative genetic parameters such as heritabilities, genetic correlations and plasticities of traits that are relevant to microevolutionary responses to environmental change. Unfortunately, quantitative genetic approaches have one major drawback – they cannot identify the actual genes responsible for genetic variation. Therefore, it is impossible to link evolutionary responses to a changing environment to molecular genetic variation, making our picture of the process incomplete. Many of the best long-term studies have been conducted in passerine birds. Unfortunately genomics resources are only available for two model avian species, and are absent for bird species that are studied in the wild. I will fill this gap by exploiting recent advances in genomics technology to sequence the entire transcriptome of the longest running study of wild birds – the great tit population in Wytham Woods, Oxford. Having identified most of the sequence variation in the great tit transcriptome, I will then genotype all birds for whom phenotype records and blood samples are available This will be, by far, the largest phenotype-genotype dataset of any free-living vertebrate population. I will then use gene mapping techniques to identify genes and genomic regions responsible for variation in a number of key traits such as lifetime recruitment, clutch size and breeding/laying date. This will result in a greater understanding, at the molecular level, how microevolutionary change can arise (or be constrained).