Because the maintenance of proper developmental temperatures during avian incubation is costly to parents, embryos of many species experience pronounced variation in incubation temperature. However, the effects of such temperature variation on nestling development remain relatively unexplored. To investigate this we artificially incubated wild blue tit (Cyanistes caeruleus L.) clutches at 35.0 °C, 36.5 °C or 38.0 °C during two thirds of the incubation period. We returned clutches to their original nests prior to hatching and subsequently recorded nestling growth and resting metabolic rate. The length of the incubation period decreased with temperature whereas hatching success increased.. Nestlings from the lowest incubation temperature had shorter tarsus length at two weeks of age, but body mass and wing length were not affected by temperature. In addition, nestlings from the lowest temperature had a significantly higher resting metabolic rate compared to mid and high temperature nestlings, which may partly explain observed size differences between groups. These findings suggest that nest microclimate can influence nestling phenotype, but whether observed differences carry over to later life-history stages remains unknown.