Daily weight increments of nestling Great Tits are expressed as ratios of observed increment divided by the increment expected under favourable conditions. We used this ratio to examine the effects of local environmental conditions on nestling growth. We demonstrate a positive relationship between nestling growth and food availability at that time and location. This relationship is stronger with the maximum rather than the median prey availability for the three to five trees sampled at a location. Residuals of the regression of the realized growth ratio on age are used to demonstrate a 10 to 20% reduction of growth on days with some daytime rain above one millimeter per hour, relative to growth in the same brood on dry days. Our trapping of the adults had an effect on nestling growth similar to that of two to three hours of rainfall. Finally, we show significantly later fledging in broods with slow growth.