Current evolution opinion is that non-avian dinosaurs were the ancestors of modern birds. However, once again we see, that that idea does not stand up to scrutiny. "The life history of non-avian theropods differed substantially from extant [existing, modern] birds".
"The timing of sexual maturation in non-avian dinosaurs is not known. In extant squamates and crocodilians it occurs in conjunction with the initial slowing of growth rates as adult size is approached. In birds [so called] living dinosaurs, on the other hand, reproductive activity begins well after somatic maturity. Here we used growth line counts and spacing in all of the known brooding non-avian dinosaurs to determine the stages of development when they perished. It was revealed that sexual maturation occurred well before full adult size was reached—the primitive reptilian condition. In this sense, the life history and physiology of non-avian dinosaurs was not like that of modern birds.
Our findings point to somatic and sexual maturity occurring simultaneously among brooding Deinonychosauria and Oviraptoridae, the two clades of non-avian dinosaurs considered most closely related to birds (Norell et al. 2006; figure 1). Viewed in a broader phylogenetic context it can be inferred that this condition probably characterized the non-avian dinosaur radiation as a whole.
Our findings also show that, despite the presence of many modern avian anatomical features such as hollow bones and feathers (Gauthier & Gall 2001; Currie et al. 2004), the life history of non-avian theropods differed substantially from extant [existing, modern] birds."
(Gregory M Erickson1,2,*,Kristina Curry Rogers3,David J Varricchio4,Mark A Norell2 andXing Xu5)
"Growth rates of pterosaurs once they hatched varied across different groups. In more primitive, long-tailed pterosaurs ("rhamphorhynchoids") such as Rhamphorhynchus, the average growth rate during the first year of life was 130% to 173%, slightly faster than the growth rate of alligators. Growth in these species slowed after sexual maturity, and it would have taken more than three years for Rhamphorhynchus to attain maximum size. In contrast, the more advanced, large pterodactyloid pterosaurs such as Pteranodon grew to adult size within the first year of life. Additionally, pterodactyloids had determinate growth, meaning that the animals reached a fixed maximum adult size and stopped growing."