Flight is a very demanding task for avians both physically and anatomically. Everything about a birds anatomy is designed around flight and small injuries can ground a bird for good. Lot's of things go into flight; feathers, wings, flight muscles, hollow bones, efficient lungs, compact torso, short tail an aerodynamic body contour. But for birds without feathers none of the other adaptations would be useful (aside from the lungs, those are great).
Flightless birds forgo these adaptations almost completely, making it near impossible to regain flight. Ostriches and emus have thick bones for running and their wings are vestigial. In the case of kiwis their feathers become more akin to fur.
However even in birds that don't always fly sexual selection comes to the rescue. Plenty of bird species have excessive plumage to impress females. Peacocks and birds of paradise for example. Which is my justification for the following idea:
My birds don't fly, they've completely adapted to a terrestrial lifestyle. However! They have plumage around their body like an umbrella or parachute. This is a result sexual selection and was later repurposed to let them fall safely from heights. What I'd like to know is how wide the feather collar would have to be for this to be possible.
I'd love a scale of weight to the required surface area to glide safely. Kiwi birds grow up to 14-18 inches (35-45 cm) and weighs 4.3 lbs. (0.8-1.9 kg). Ostriches are the largest living bird, an adult male may be 2.75 metres (about 9 feet) tall and weigh more than 150 kg (330 pounds). If bigger birds could do this, I'd love to know.