For decades now, the drumbeat of dinosaur news has been their similarity to birds. They were warmblooded! They had feathers! And they’re still around, because birds are actually dinosaurs.

All true, but those that were nonavian dinosaurs, as they are now called, were not all beak and tweet. They were closely related to other living reptiles like crocodiles, and new findings about how long their eggs took to hatch bring that point home.

Scientists reported on Monday that by using a new technique on exceedingly rare fossils of unhatched dinosaur embryos, they determined that those embryos took twice as long to hatch as bird eggs of a similar size. The embryo of a large duck-billed dinosaur took at least six months to hatch, and the eggs of larger dinosaurs may have taken even longer.

The long incubation times complicate thinking about dinosaur behavior. While some kinds of dinosaurs may have tended their eggs and young, for others the difficulty of hanging around for most of a year to watch buried eggs would have been too much. And long incubation times mean slow reproduction, a definite disadvantage when a comet or asteroid slams into the planet, as happened 65 million years ago, leading to the extinction of dinosaurs and many other species. But not birds.