Accuracy in Determining the Age of Nestling Red-Tailed Hawks
Because raptor siblings are only a few days apart in age, techniques that accurately estimate age are important when studying their growth and survival. Age estimators based on comparisons of young to photographs and weight curves of known-age chicks have been proposed (Hoffmeister and Setzer 1947, Austing 1964, Austing and Holt 1966, Luttich et al. 1971, Moritsch 1983). Due to the subjective nature of photographic comparisons and variation in nestling weight resulting from changes in local conditions and digestive tract contents, neither method seems accurate enough for estimates within a few days of age.
Bechard, Marc J.; Zoellick, Bruce W.; and Nickerson, Michael. (1985). "Accuracy in Determining the Age of Nestling Red-Tailed Hawks". The Journal of Wildlife Management, 49(1), 226-228. http://dx.doi.org/10.2307/3801875