First egg dates for great tits (Parus major) and blue tits (Cyanistes caeruleus) breeding in three deciduous woods in Cambridgeshire, England in 1993 to 2016

Field data collected by trained and experienced personnel. Egg dates have been checked against original paper records. Numbers of lines of data have been checked and data values have been checked for outliers. Individual egg date values have been checked against mean values per year for each site.