 A histological study by Erickson, Norell, Zhongue, and others in 2009 estimated that Archaeopteryx grew relatively slowly compared to modern birds, presumably because the outermost portions of Archaeopteryx bones appear poorly vascularized; in living vertebrates poorly vascularized bone is correlated with slow growth rate. They also assume that all known skeletons of Archaeopteryx come from juvenile specimens. Because the bones of Archaeopteryx could not be histologically sectioned in a formal skeletochronological (growth ring) analysis, Erickson and colleagues used bone vascularity (porosity) to estimate bone growth rate. They assumed that poorly vascularized bone grows at similar rates in all birds and in Archaeopteryx. The poorly vascularized bone of Archaeopteryx might have grown as slowly as that in a mallard (2.5 micrometres per day) or as fast as that in an ostrich (4.2 micrometres per day). Using this range of bone growth rates, they calculated how long it would take to "grow" each specimen of Archaeopteryx to the observed size; it may have taken at least 970 days (there were 375 days in a Late Jurassic year) to reach an adult size of 0.8 – 1 kg (1.8 – 2.2 lb). The study also found that the avialans Jeholornis and Sapeornis grew relatively slowly, as did the dromaeosaurid Mahakala. The avialans Confuciusornis and Ichthyornis grew relatively quickly, following a growth trend similar to that of modern birds. One of the few modern birds that exhibit slow growth is the flightless kiwi, and the authors speculated that Archaeopteryx and the kiwi had similar basal metabolic rate.
