Reproductive System and Behaviour
Dinosaurs and birds share many reproductive similarities in their means of producing eggs, and the behaviour involved in doing so. Many fossils show that Maniraptorian dinosaurs were similar to birds in the way they lay their eggs and also what the eggs look like. Dinosaur eggs and bird eggs are both oval in shape, and contain the same shell composition. The discovery of a female dinosaur fossil with two eggs inside further suggests a dinosaur-bird linkage, as generally reptiles lay a large number of eggs while birds lay a very small number. Dinosaurs were known to lay their eggs in nests, with some dinosaurs discovered to have nesting colonies, and studies suggest they may have returned to the same place each year just as birds do.
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Two main reproductive behaviours tie dinosaurs to birds, brooding and mixed parental care. The Oviraptor from Mongolia was once thought to be an egg snatcher, but specimens have since been discovered in a crouched position over nests of eggs in the same fashion you would see a bird. Studies suggest that the leg bones of dinosaur hatchlings were weak and the babies couldn't stand for some time after hatching, this implies a large degree of parental care as the hatchlings simply couldn't survive on their own. The meat-eating dinosaur Troodon was among many dinosaurs that implemented male parental care. The male Troodon would guard the nest and brood the eggs while the female went to forage for nutrient-rich meals that would better advance their reproductive success. The nests may have even contained eggs from multiple females, a trait seen in ostriches today. Parental care and mixed parental care is seen in 90% of birds today, further strengthening the link between dinosaurs and birds.
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