Teaching birds to fly for forage

[LocoPollo]

My game birds have always flown over the fence to forage throughout the day, and then the fly back over into the run and back into the coop at night. I like this because it lets them forage and eat all the bugs, etc. (and of course cheaper on the feed!) The other breeds that I have gotten since have yet to fly out. Is there a way to teach them? Are they even able (I have ee's and speckled sussex)?

 

[beakkeeper]

They might just not have the build for it.