Nest building requires parental investment and we should expect to find trade-offs between building and other behaviours or demands. For example, species breeding in temperate zones face the seasonal shift from spring to summer and might alter their nest construction in relation to environmental conditions as the season progresses. We quantified seasonal variation in the length of the nest building period, nest mass, composition, Hen Flea Ceratophyllus gallinae abundance and egg production in nestbox breeding Blue Tits Cyanistes caeruleus. The nest building period decreased seasonally, possibly indicating individual adjustment of optimal time of reproduction. Despite this, there was no seasonal trend in overall nest mass, although nest composition did change. The mass of the nests' moss base and Hen Flea abundance showed no seasonal variation, but there was a seasonal decline in the mass of the cup lining material, suggesting that female Blue Tits selectively adjust the cup lining component of their nests to reflect increasing ambient temperatures in order to create an optimal microclimate for the developing embryos and nestlings.