Why Do Birds Lay Eggs?

oviparous, viviparity, bird, birds, egg, eggs, animal, reproduction, embryo, embryos, embryonic development, turtle, turtles
Scientists believe some animals still lay eggs for various reasons. (Image credit: Heiko Schmaljohann.)

Most newborn mammals, some reptiles and even some fish emerge swimming or generally flailing about. But baby birds and turtles are stuck cracking open eggshells.

Oviparous animals produce eggs that hatch after leaving the mother’s body.

Mammals and other groups have evolved the ability to keep embryos within the reproductive tract until development is complete. One of the challenges of this baby-making method called viviparity is that babies are heavy to carry around.

Scientists suggest viviparity has yet to evolve in any species of bird because mother birds can’t fly with weighty developing embryo passengers.

A second problem with viviparity is getting air to the egg.

The eggshells and oviducts of birds and turtles, unlike those of other reptiles, do not allow much oxygen exchange to occur inside the mother, according to a 2000 study published in the journal BioScience. This constricted airflow may prevent long-term embryonic development from happening in mother birds and turtles.

The theory suggests that bird embryos get more air and develop into healthier chicks laying in a nest than they would inside the mother's body.

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Corey Binns lives in Northern California and writes about science, health, parenting, and social change. In addition to writing for Live Science, she's contributed to publications including Popular Science, TODAY.com, Scholastic, and the Stanford Social Innovation Review as well as others. She's also produced stories for NPR’s Science Friday and Sundance Channel. She studied biology at Brown University and earned a Master's degree in science journalism from NYU. The Association of Health Care Journalists named her a Centers for Disease Control and Prevention Health Journalism Fellow in 2009. She has chased tornadoes and lived to tell the tale.