Complete data collected from 2009-current on a nest box population of blue tits near Edinburgh, UK. It represents a lot of work by many people and has been funded by NERC and The Royal Society of London. The data are freely available without restrictions. We hope that users respect the fact that some of these data constitute unpublished work of PhD students. A strong motivation for continued data collection is that there is priority for these students to use these data to address the questions in which they are interested. We therefore encourage users to contact us regarding research plans, to formally preregister their analysis plans prior to data exploration, and submit analysis scripts upon publication. Preregistration is an effective tool for combatting high type I error rates, and ensures that the data we have worked hard to collect is used to generate robust scientific findings. Submission of analysis scripts allows us to validate earlier results as new data are collected.