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Songster
One of the posts I read here on BYC indicated the clear plastic flapper was removed and replaced with slightly lighter-duty, flexible, clear plastic cut into three strips. The person pinned two of the flap strips up for a week, so the birds could shoulder their way through the doorway. Then a second section was allowed to fall into place for another week. The chickens pushed their way through, and when the third strip was released on the third week, the chickens had learned to push the flaps apart to enter or exit through the doggie door.
That method might work. I would be sure to find plastic that was very lightweight and flexible, though....