We addressed the question of why juvenile R. ferrumequinum that are born early (May to early June) in the nursery roost at Stackpole, West Wales, tend to have a higher forearm growth rate than individuals born later (late June to July). An appropriate nonlinear growth equation (either logistic, Gompertz, von-Bertalanffy, or asymptotic regression) was identified to fit the measurements of the forearms of juvenile R. ferrumequinum collected from marked individuals at the roost between 1994 and 2013. The logistic growth model provided the best line of fit to the changes in the forearm lengths, indicated by the lowest standard error. Over the 20-year study period, the mean values of the logistic growth parameters were asymptotic length (L∞) = 55.10 mm and growth constant (K) = 0.11 day^-1. Multiple regression analysis indicated that the most significant predictors of the variance in L∞ and K (R² = 58.7% and 54.3%, respectively) were the nursery roost temperatures at midnight, and the total foraging times of the adult bats during the night. If the juveniles were born early, when the roost temperature was colder, and the foraging times of the adults were shorter, then a lower value of K produced forearms that grew toward a longer L∞. If the juveniles were born late, when the roost temperature was warmer, and the foraging times of the adults were longer, then a higher value of K produced forearms that grew toward a shorter L∞. Reasons are proposed to explain the differences between the growth of early and late born individuals, including the effect of increased body mass associated with warmer roost temperatures and possible causes of longer foraging times.