Eurasian Blue Tits (Cyanistes caeruleus) time reproduction so the period for rearing chicks is synchronized with the peak of abundance of caterpillars, most of which feed on young leaves of deciduous trees. A long-term study at Silwood Park aims to understand the extent to which a mismatch between breeding phenology and prey availability affects a wild population of these birds. About 230 nest boxes and a thousand oak trees distributed across the campus woodlands are monitored annually in spring since 2002 to track the breeding activity of nesting Blue Tits and record the time of emergence of new Oak leaves. The study site is Silwood Park Campus from Imperial College London, Buckhurst Road, Ascot, Berkshire SL5 7PY, United Kingdom. Silwood Park campus, with about 70 ha, contains ancient woodlands and few-decades-old oak-dominated woodlands. Study oak trees have been set across the campus woodlands, which are classified as W10a, W10e and W16a using the National Vegetation Classification. The study site experiences an average annual rainfall of 698mm with little seasonal pattern (1987-2022). Mean hourly temperature is 10 oC with July max of 23 oC and January min of 1.4 oC (1987-2022) ( https://zenodo.org/records/14680305) This dataset contains infromation of Blue Tit brood history, parents and chicks starting 2002. Details on data collection can be found in the metadata file. This is an ongoing study. Last update December 2025 General experimental design: There is a network of woodcrete nest boxes across the campus woodlands used to study the breeding phenology of Blue Tits. Most boxes have a small entrance (26 mm) but two of the woodlands have also a set of boxes with a larger entrance (32 mm). Boxes with larger entrances can be used by Great Tits and all nest boxes are occasionally occupied by Nuthatches and Coal Tits. Nest boxes have a unique name with a letter and a number. Boxes of the same entrance size and within the same woodland have the same letter (also known as territory). The Number of nest boxes have varied throughout the study. Nest boxes with letters A-H were erected in 2001 and those J & K in 2004 (Henderson_UGthesis_2005). Nest boxes with larger entrance (X & GA) were added in 2009 (Bell_PhDThesis_2014). Several have been excluded from the experiment due to the sale of campus land or due to unknown reasons (information loss), like all nest boxes in territory K. Typically nest boxes are moved to a neighbour tree when the host tree is damaged or fallen. In 2022 breeding season there were 224 active nest boxes, 176 boxes have a small entrance (26 mm) that exclude Great Tits and 48 have a larger entrance (32mm). Data Collection: Regular visits to nest boxes start on 1st April (day 1). Monitoring aims to record the stage of nest building, the date when first egg is laid, number of eggs and successful chicks fledging a nest. Females, and occasionally males, breeding in nestboxes are captured, measured and ringed during egg incubation, during the feeding phase (e.g. day 14) or both. Adults and chicks are fit with a ring with a unique number issued by the British Trust of Ornithology (BTO). The BTO data base has also a copy of data of all birds measured, recaptured and ringed for this project. At least from 2021 this data contains nest box location. Data available, Records of nest boxe use throughout the breeding season (2002-2025) Biometrics of female and chicks (2002-2025) Nest building phenology (2022-2025) , compostition (2022, 2024, 2025) and parasite load (2024, 2025) Record of nest manipulatoin s and samples taken form individuals Food availability to chicks assessed by frass and caterpillar abundance (2024, 2025)