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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.

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  • $\begingroup$ Love the topic.. but in this case difficult to say anything about it, without knowing the motivation to fly and the preferred method of takeoff.. 1) would your bird use flight to escape predators ? or does it just happen for recreation, when the bird feels like it ? related 1a) does it need to jump off a cliff or mountain to set in flight ? Does it use trees to takeoff ? 1b) Does it need sustained gliding flight, or just survive a fall ? $\endgroup$
    – Goodies
    Sep 24 at 19:23
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    $\begingroup$ @Goodies To get to the other side. 1) Mostly to impress mates. 1a) It climbs and jumps to glide down safely. 1b) Surviving a fall. $\endgroup$ Sep 24 at 20:02
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(I took the title question as the lead for this.. the bird should survive a fall)

Center of gravity concerns with parachuting

In terms of physiology, for a sustained glide or parachuted fall, you'd need a way to place the center of gravity symmetric i.r.t. the upward airflow. Else you'd go spiral down.

E.g. peacock feathers would make a great parachute.. but the peacock should be lean and mean, preferably a bit taller, to be able to bend backward, and shift its center of gravity to the middle of its "parachute".

enter image description here https://www.ndtv.com/india-news/peacock-indias-national-bird-could-be-termed-vermin-in-goa-1276656

Ostriches

Let's hand-wave weight for a moment, focus on shape and parachute-function.. Ostriches have this center of gravity problem too, but to a lesser extent. They have plenty of feathers, and when it is mating, it will put the feathers in a nice mantle shape around the body. Good parachute.. but the ostrich also has a long neck.. it would have to extend its feathers and bend the neck backward when falling. The center of gravity may land in a "sweet spot" where the bird would be able to control the glide as well. Many birds are able to bend their neck backward:

enter image description here https://www.dreamstime.com/photos-images/ostrich-mating.html

The ostrich could also stretch the neck forward, and fall with its but down, like in this posture,

enter image description here

Parachute size needed is proportional to weight

In any case, ostriches are really too large.. legs are in the way.. for an ostrich to really fall safely, it would need much more and preferably wider feathers ! Ostriches have human weight range (65-142kg) so the parachute or d-wing should approach the size of a parachute we use. Peacocks have it easier, they are lighter,

enter image description here https://www.rocketreviews.com/index.php

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  • $\begingroup$ The lack of flight muscles means they can’t be top heavy, as most of their muscle ends up in the legs. Strong long legs will also be good at cushioning a fall. $\endgroup$ Sep 24 at 20:06
  • $\begingroup$ Absolutely. The ostrich has some advantage there.. but I'm afraid it would not be able to stabilize easily, because of the weight of these legs. To land safely, it should be able to position the legs properly, just before touchdown. $\endgroup$
    – Goodies
    Sep 24 at 20:10
  • $\begingroup$ I've finished, added a handy link for the parachute size. Good luck with the other answers ! Looking forward to options for chicken, pinguins, kiwi birds.. $\endgroup$
    – Goodies
    Sep 24 at 20:16
  • $\begingroup$ The “parachute” has to be on top and the center of gravity underneath. A solution is for the “parachute” to be around the neck. The bird tucks it’s legs in when falling, extends them at the end of the fall. $\endgroup$ Sep 24 at 20:17
  • $\begingroup$ In creature design, you could do that. Earth birds - like above examples - have lost flight. That means their feathers are on the forelimbs. That counts for all Earth birds, as far as I know.. there exist birds with neck feathers, but they'll always have forelimb feathers too. $\endgroup$
    – Goodies
    Sep 24 at 20:20

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