Nestling growth may be used to estimate age of nestling raptors, which is valuable for investigating hatch order dynamics and nestling behavior, as well as assessing reproductive rate and back-calculating hatching date. To estimate nestling age, the most valuable parameter to measure growth is one that does not vary greatly with environmental factors, and ideally is applicable over a wide range of populations. We measured growth of nestling Red-shouldered Hawks (Buteo lineatus) in Quebec, Canada, from ages 3 days to near fledging (38 days old), and compared growth of several parameters in different size broods. As a validation study, we measured similar parameters one time in known-age nestling Red-shouldered Hawks in southwestern Ohio. Growth rates for tarsus length, bill length, and tail length differed between nestlings in broods of one and three young, respectively, in Quebec. However, mass gain and growth of secondary feathers (mean length of first and second secondaries) did not differ between brood sizes, although mass gain was more variable than secondary growth. These results suggested that secondary feather length was the most valuable parameter for estimating nestling age in Red-shouldered Hawks. Comparing Ohio nestlings to Quebec nestlings, we found that growth of secondary feathers differed significantly, with Ohio nestlings having smaller secondary length, relative to age estimate. Application of the equation generated with the Quebec data to estimate the age of the Ohio nestlings based on secondary length resulted in estimates that were 2.3 ± 0.3 days (range 0.25–4.5 days; n  =  22) younger than the Ohio nestlings' actual ages. Based on this validation study, we suggest that the use of the Quebec age-secondary length equation to estimate age for nestling Red-shouldered Hawks in other parts of eastern North America is acceptable, though with the caveat that such estimates are associated with potential small errors.