Twisting and bending deformities of the long limb bones of growing birds are common. In domestic poultry they are associated with rapid growth. In order to examine the variation in bone growth rates amongst the Class Aves as a step towards learning about the aetiology of growth deformities, we collected data on this and several other variables from a large sample of species. Adult tarsometatarsal length (ATL) was proportional to adult weight (W)0.36. The exponent did not differ significantly from 1/3. Mean legginess (L), defined as ATL/W1/3 (i.e. ATL.W-1/3), was 80 mm.kg-1/3 but varied according to the habitat occupied; terrestrial and arboreal birds had relatively longer legs than aquatic and aerial ones. ATL growth rate (GR) averaged 20 mm.d-1 and was not correlated with W. It was highly influenced by pattern of development, being on average about three times greater in altricial compared with precocial birds of the same adult weight. Amongst birds of the same developmental pattern, GR was linearly related to L. Tarsometatarsal growth rate varied from 0.35 to 60 mm.d-1 in the Class Aves and there was no evidence of intrinsic limits to GR from our results. How rapidly such bone growth rates are achieved remains to be determined.