aperture of a large specimen (DNHM-D 2454) indicates a maximum egg diameter of . In modern birds, proportionally large eggs are commonly found in species whose hatchlings do fully depend on their parents (altriciality), while smaller eggs are often found in species whose hatchlings are more developed and independent (precociality). As the estimated egg of the specimen would have been around 30% smaller than expected for a modern altricial bird, it is likely that Confuciusornis was precocial. A 2018 study by Charles Deeming and Gerald Mayr measured the size of the pelvic canal of various Mesozoic birds including Confuciusornis to estimate egg size, concluding that eggs would have been small in proportion to body mass for Mesozoic birds in general. These researchers further posit that an avian-style contact incubation (sitting on eggs for breeding) was not possible for non-avian dinosaurs and Mesozoic birds, including Confuciusornis, as these animals would have been too heavy in relation to the size of their eggs. Kaiser, in 2007, argued that Confuciusornis likely did not brood in an open nest but might have used crevices in trees for protection, and that the small size of the only known egg indicates large clutch sizes. In contrast, a 2016 review by David Varricchio and Frankie Jackson argued that nesting above the ground evolved only at a much later stage, within Neornithes, and that Mesozoic birds would have buried their eggs on the ground, either fully or partially, as seen in non-avian dinosaurs. Growth Growth can be reconstructed based on the inner bone structure. The first such study on Confuciusornis, presented by Fucheng Zhang and colleagues in 1998, used scanning electron microscopy to analyze a femur in cross section. Because the bone was well vascularized (contained many blood vessels) and showed only a single line of arrested growth (growth ring), these authors determined that growth must have been fast and continuous as in modern birds, and that Confuciusornis must have been endothermic. Zhang and colleagues corroborated this claim in a subsequent paper, stating that the bone structure was unlike that of a modern ectothermic alligator but similar to the feathered non-avian dinosaur Beipiaosaurus. However, these authors assumed that endothermy in Confuciusornis had evolved independently from that seen in modern birds. This concurred with earlier work by Anusuya Chinsamy and colleagues, who described distinct lines of arrested growth and low vascularity in other Mesozoic birds that are more derived