Analyses of fossil eggshells suggest that duck-billed dinosaurs (depicted at left) buried their eggs in mounds of vegetation like today’s crocodiles, whereas evolutionarily advanced, more birdlike dinosaurs such as oviraptorids (right) incubated their eggs in an open nest.

The links between dinosaurs and birds keep getting stronger: skeletal structures, feathers—and now nests. Whereas some dinosaurs buried their eggs crocodile-style, a new analysis suggests that other dinosaurs built open nests on the ground, foreshadowing the nests of birds.

Interpreting the fossil record is always tough, but analyzing trace fossils such as nests is especially daunting. Those structures, and the materials used to make them, usually aren’t preserved, says Darla Zelenitsky, a paleobiologist at the University of Calgary in Canada. When paleontologists do find a nestlike structure that includes material such as sticks or other vegetation, the question arises: Was this stuff part of the original nest, or just carried there with the sediment that buried the nest and helped preserve it?

To gain insight into dinosaur nesting habits, Zelenitsky and her colleagues studied the most durable parts of nests—the eggs themselves. (Being largely made of the mineral calcium carbonate, they’ve got a head start on fossilization and are sometimes incredibly well preserved.) In particular, the team looked at the size and arrangement of small pores in the ancient shells, because those details are telling in modern creatures.