[mr_toad]

If there were one ton chickens running around they’d probably be killing us. They’re not that far removed from their ancestors.

[yesenadam]

There were half-ton "chickens" in Australia 5 million years ago. 10 feet tall!

https://en.wikipedia.org/wiki/Dromornis_stirtoni

Or the even heavier Elephant birds in Madagascar until 800 years ago.

https://en.wikipedia.org/wiki/Elephant_bird