How are some ducks flightless?

[Jason_laurin613]

I understand there are flightless ducks like runners and cayuga. But how? Were they bred that way or is it their proportions?

 

[SolarDuck]

It's because they are bred for traits other than flight, mostly meat. When the birds are too heavy they just can't fly.

 

[igorsMistress]

Runners have been bred and selected over the years for that trait. They were herded to the rice fields to forage for slugs and such, then herded home at night. Some are too heavy. For some it’s possibly due to evolution brought about by environmental factors. It depends.