In hatchlings, Crs is several folds higher than in neonatal mammals of comparable body size. At first glance, this could imply that active expiration is necessary to overcome a potentially long τrs. However, the current data show that this does not have to be the case, because the hyperinflation lowers Crs and τrs to values compatible with passive expiration during resting breathing. Incubation in 15% O2 did not modify appreciably the static and dynamic mechanical properties of the hatchling's respiratory system, seemingly in contrast with previous results in neonatal mammals following hypoxic gestation. This could indicate that hypoxia affects specifically the development of the lung tissue, not that of the air sacs and the chest wall, which in avian hatchlings represent the major contributors to the mechanics of the respiratory system.