Avian growth has been traditionally examined using logistic, Gompertz or von Bertalanffy non-linear equations. Although many studies have analysed nestling mass increase and the factors affecting growth rates, there is little information on nestling growth curves for several avian groups, such as Nuthatches (Sittidae). Moreover, the breeding biology of Nuthatches is still poorly studied because of their reluctance to use nest boxes. Here we examined the growth pattern in Eurasian Nuthatch Sitta europaea nestlings, in addition to studying the breeding biology of this species, in a mixed deciduous forest from Central Portugal during one year. The nestling growth was well fit by the logistic and Gompertz models, whilst the von Bertalanffy equation was less accurate, especially when predicting the asymptotic mass and the mass in the first development stages. Nestlings attained their maximum mass around the age of 18 days, while the largest daily mass increase was achieved when nestlings were 8 days old. Weather conditions did not affect nestling growth. Regarding the breeding biology, the mean date of laying of the first egg was 3 April, the mean clutch size was 5.43 ± 0.78 (±SD) eggs and the mean number of fledglings per clutch was 4.00 ± 2.09. The proportion of eggs that produced fledglings in a nest averaged 72%. Clutch size and number of fledglings were negatively correlated with the laying date. These results contribute to our understanding of Eurasian Nuthatch breeding biology and nestling development, the latter providing useful data for the standardization of ornithological practices, such as determining the appropriate fledgling ringing period.