The emu and ostrich can't fly because their ancestors became fat and lazy when the dinosaurs died

Birds like the emu and ostrich gave up on flight because they becamefat and lazy when the dinosaurs died, according to new research.

An abundance of food and a lack of predators following the extinction of dinosaurs saw a number of birds fatten up and simply stop bothering to take to the air.

Many scientists had believed that the ancestors of modern flightless birds - such as emus and ostriches - were also flightless.

But a new study suggests they could fly and gradually stopped making the effort.

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