Studies investigating nest function in birds show that individual preferences and environmental temperature can affect the type and amount of materials used in their construction and, thus, how well insulated they are. Levels of insulation of bird nests may be important because this could impact on heat loss by adults and eggs during incubation, and by nestlings during rearing, which may in turn affect individual fitness. Here we used infrared (IR) thermography to measure the surface temperature of nests of Great Tits Parus major and Blue Tits Cyanistes caeruleus in situ during incubation to test the hypothesis that nest insulation predicts reproductive success. Previous studies of thermodynamics during incubation have focussed on factors, e.g. egg temperature, which do not directly measure the thermal conditions in the nest itself. Rarely applied to studies of avian incubation, IR thermography has yet to be used to quantify the thermal properties of nests with the incubating bird in situ. The rate of temperature change (°C/cm) of the nest material, as determined for the first 1.5 cm away from the edge of the bird, was significantly associated with fledging success in Blue Tits, although not Great Tits. This provides the first evidence that the insulatory properties of nests during incubation can correlate with offspring fitness, and so has important implications for the study of nest function in an ecological context. IR thermography provides a methodology that allows future research to investigate the factors that determine nest insulation.