Paleontologists at Montana State University think that the nesting habits of some Mesozoic-era dinosaurs bear resemblance to the nesting habits of today's birds, providing further evidence that dinosaurs were the ancestors of birds.
In case it's not at your fingertips, the Mesozoic era was a period of evolution between 230 and 65 million years ago. This period of earth's history was characterized by the appearance of dinosaurs and flowering plants. Birds of a sort existed during the Mesozoic, the most abundant being the Enantiornithines. Like today's birds, they had feathers. They partially buried their elongated eggs. (What we think of as "egg-shaped" eggs didn't appear until much later, with modern birds, and it was modern birds that began incubating them in nests off the ground.) There were also birdlike dinosaurs, among them Troodontids, or Troodon formosus. These weighed about 100 pounds and had serrated teeth. They laid hard-shelled eggs like modern birds, and they didn't bury them completely for incubation like reptiles did (and still do).
MSU paleontology colleagues David Varricchio and Frankie Jackson published a paper in August in The Auk: Ornithological Advances in which they examined the evolution of bird reproduction. The point to note: "Reproduction in modern birds is distinct among living vertebrates and many aspects of this (modern bird) reproduction mode trace their origin to (Mesozoic-era) theropod dinosaurs...but not really beyond them to more distantly related dinosaurs." In other words, reproduction links modern birds only to the most birdlike dinosaur species, which means that the latter might well have been the precursor to today's birds.
Varricchio and Jackson published their work in The Auk, an international journal pertaining to birds, and not in a paleontological publication, in order to work toward a consensus that has divided scientists for almost two centuries. "People have argued about the bird-dinosaur connection since the 1800s," says Varricchio. "But, since then, there has been overwhelming skeletal evidence [to support the connection]. Then in 1996, we learned that some dinosaurs had feathers. Well, their reproduction follows that pattern, as well."
Why did some dinosaurs have feathers? Why did some of them incubate their eggs without burying them fully? No-one is sure yet. But what is known is that modern birds are all we have have left of a world once populated by dinosaurs.