Jack Horner gestures at rows of steel cabinets and curtained shelves of fossils. "They're all just a bunch of bones," he shrugs, as if apologizing for the fact that the collection he's built over almost two decades doesn't roar and prowl like a Hollywood T. rex. But this silent bone vault at the Museum of the Rockies in Montana tells more about the lifestyles and social behaviors of dinosaurs than all the booty of the 19th-century bone wars combined.

On a tip from a rural Montana rock shop proprietor in 1978, Horner and his collaborator Robert Makela found their first dinosaur nest in an upland Cretaceous formation far from the heavily quarried coastal deposits. Three seasons later, Horner's crews had excavated a total of seven more nests, all occupied in the same year and spaced to allow a twenty-three-foot-long adult Maiasaura to tend each nest at the same time. After a century of mystery, it was clear that at least some dinosaurs nested in socially gregarious colonies like birds, not crocodiles.

Digs at another site unearthed an enormous rookery of a second new duckbilled dinosaur, Hypacrosaurus stebingeri. Laboratory studies of both species showed that a hatchling's bones had a very high proportion of soft cartilage, making these babies weak and helpless at birth. The parents must have fed and protected them in the nest for an extended period.

Furthermore, baby skeletons of the small carnivorous theropod Troödon were much more calcified and stronger, suggesting that young Troödon were capable of leaving the nest sooner, perhaps almost immediately upon hatching.

Horner was particularly gratified when Mark Norell of the American Museum of Natural History returned to the Flaming Cliffs in 1993 to unearth an Oviraptor skeleton unmistakably positioned on a nest exactly like a brooding hen. The dinosaur that Henry Fairfield Osborn had literally labeled an egg stealer was really an egg incubator, and more like a bird in behavior and physique than a previous generation had been willing to admit.