1. Fledging weight has been shown to correlate with survival in many bird species and, therefore, is an important component of fitness. Fledging weight results from growth during the nestling stage. Hence, environmental effects on nestling growth in altricial bird species play a key role in proximate explanations of selection after fledging. 2. We describe and test a method to quantify environmental effects on (daily) increments in nestling weight. It accounts for the known genetic component of growth and simultaneously detects the effects of short-term environmental fluctuations on growth. 3. The method is illustrated with data on daily weights of individual great tit nestlings from a rich and from a nutritionally marginal study area near Basel, Switzerland. However, the method can also be applied to other traits. 4. The observed weight increment is expressed as a ratio through division by the increment expected under good conditions. The expected increment is calculated from a Richards growth curve with a shape parameter derived from a fit on nestlings growing under favourable conditions and an asymptotic weight based on parental weight to incorporate the genetic component. 5. Under good conditions the ratio of realized over expected growth increment is on average close to 1.0 irrespective of nestling age. Under poor conditions, however, there is a decrease in ratio with an increase in age due to a larger influence of the environmental conditions later in the growth period. 6. The method is only useful when the expected growth increments are greater than the measurement error, in our examples about 0.1 g. 7. Monte Carlo simulations confirm that our interpretations are realistic. 8. We demonstrate that mean parental winter weight as an estimator of the asymptote is a practicable way of incorporating genetic variance in final body weight into growth models.