November 13 , 2016:

Birds are the only extant tetrapods, which incubate their eggs with body heat in nests outside of soil. Nesting free of sediment is now considered to have evolved comparatively late in avian evolution, within Ornithuromorpha, the clade including modern birds. Egg turning by the breeding adults was identified as an evolutionary corollary of this derived reproductive behavior and is due to a higher albumen content of the egg. An associated increase of egg width in the ornithuromorph subclade Ornithurae is reflected by opening of the pubic symphysis. Here it is hypothesized that the lower yolk to albumen ratio led to a reduced hatchling precocity and that different habitat preferences of Mesozoic birds account for the evolution of the derived avian reproductive biology in Ornithuromorpha. Unlike their sister taxon, the predominantly arboreal Enantiornithes, early ornithuromorphs were aquatic or terrestrial animals. After egg deposition in the ground was abandoned, early ornithuromorphs probably still nested on the ground, and in this case increased parental care is more likely to evolve in taxa that also forage on ground level. In Enantiornithines, by contrast, the evolution of increased post-hatching parental care was impeded by the fact that the “superprecocial” chicks hatched on the ground and entered arboreal habitats soon thereafter. If an increased post-hatching parental care was among the selective drivers of ornithuromorph breeding biology, offspring predation may have been a critical factor in the evolution of Cretaceous birds.

Gerald Mayr (2016)
Evolution of avian breeding strategies and its relation to the habitat preferences of Mesozoic birds.
Evolutionary Ecology (advance online publication)
DOI: 10.1007/s10682-016-9872-1