How bird reproduction evolved

An almost <em>Oviraptor philoceratops</em> skeleton. This theropod dinosaur species almost certainly had feathers and had already evolved nest and egg incubation methods resembling those of birds. Ghedoghedo (commons.wikimedia.org).
An almost Oviraptor philoceratops skeleton. This theropod dinosaur species almost certainly had feathers and had already evolved nest and egg incubation methods resembling those of birds. Ghedoghedo (commons.wikimedia.org).
A new review has helped our understanding of how the unique reproduction methods of birds may have evolved.

Birds’ reproductive biology is dramatically different from that of any other living vertebrates, and ornithologists and paleontologists have long wondered how and when the distinctive features of bird reproduction originated. A new review has examined answers from three sources to shed new light on the subject: modern birds, fossils of primitive birds and fossils of the dinosaurs from which birds are descended.

All modern birds share certain reproductive features, such as a single functional ovary and the practice of incubating their eggs through direct contact. Analysis of the bird family tree also suggests that early birds built simple, open nests on the ground and that their young were 'precocial', meaning they were well-developed and almost ready to fend for themselves when they hatched. Those dinosaurs close to the ancestry of birds shared some of these traits, but they had two functional reproductive tracts and their eggs were smaller relative to their body size and more elongated than those of modern birds.

A prepared fossil <em>Oviraptor philoceratops</em> nest at the American Museum of Natural History, New York. The theropod got its name after it was initially thought to be predating the eggs rather than incubating them. Photo: Steve Starer (commons.wikimedia.org).

Mesozoic bird and non-avian theropod dinosaur fossils indicate that reproduction passed through five stages from basal theropods to neornithines (modern birds): pre-maniraptoran theropods, oviraptor-grade maniraptorans, troodontid-grade paravians, Enantiornithes (ancient birds with teeth and clawed wings) and basal Neornithes. Major changes occurred incrementally in egg size, shape and microstructure; in nest form; in incubation method; and in parental care. Reproduction in enantiornithine birds made use of a single ovary and oviduct, and the fossil eggs found have been planted upright within sediments. Incubation is likely to have been by a combination of sediment and attendant adult or eggs fully buried with superprecocial young (somewhat like crocodiles or turtles). Incubation modes of derived non-avian theropods and enantiornithines may have favoured paternal care similar to that seen in modern birds.

Therefore, the fossils of primitive birds and eggs from the Mesozoic era place them midway between their dinosaur ancestors and their modern descendants, with eggs between those of pre-avian dinosaurs and modern birds in term of size and shape. In this way, David Varricchio and Frankie Jackson of the Montana State University were able to trace the evolution of bird reproduction through a series of distinct stages from pre-avian dinosaurs to the birds of today.

“Reproduction in modern birds is distinct among living vertebrates. Many aspects of this reproduction mode trace their origin to theropod dinosaurs such as Oviraptors and Troodontids, but not really beyond them to more distantly related dinosaurs,” according to Varricchio. “Interestingly, reproduction in the most common group of Mesozoic birds is very similar to that of these dinosaurs, and so still differs from modern birds. Consequently, modern birds stand apart from Mesozoic birds, and perhaps this contributed to their surviving the end-Cretaceous extinction event.”

Varricchio, D J, and Jackson, F D. 2016. Reproduction in Mesozoic birds and evolution of the modern avian reproductive mode. The Auk 133: 654-684.