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I've been designing bird-like creatures that both fly and walk using their legs, saving the trouble of growing extra muscle mass for both. Because of this they have an odd look for birds: They have scrawny upper bodies sitting on top of muscular waists with feathered legs. What would be their wings have no use in flight and have instead been converted to tiny bird hands with talons. They walk on two toes like ostriches and third toe from which their primary flight feathers grow, this toe is bend upwards when walking.

Because of the way they fly their flight muscles aren't located on the torso and they don't have a keel. Their pelvis and leg muscles are adapted for the mechanical demands of flight and the muscles use the same rope-and-pulley system as ordinary birds. This gives them more room to flap their wing-legs and focuses their centre of gravity ventrally for stability during flight. My previous question was about how they fly, if you have a related comment put it there.

The problem that's been pointed out to me is that the eggs would break in the uterus due to the mechanical stress of flight. This could prove to be a problem. Any ideas? (Besides flying normally or not at all.)

EDIT: As was mentioned in the answers eggs have high compressive strength but I’m also worried about egg formation.

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    $\begingroup$ It sounds like you're trying to create a problem where none exists. Any positional movement (e.g. up, down, left, right) would probably be similar to birds with normal wings (i.e. not a problem). Any compression and such would probably be fairly easy to handwave away: everything just happens around the uterus; the uterus itself is unaffected, much like you can raise and lower your arm without ripping apart or putting too much pressure on the various veins and bones and whatnot in your chest. $\endgroup$
    – NotThatGuy
    Oct 7 at 8:19
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    $\begingroup$ @NotThatGuy it seems that you never flied by your own means. Gotta tell you, not an easy feat. But not all is lost... maybe you can take a look on the sternum of a bird and notice that's not flat, but rather like an Y-beam. "Form follow function" and then you can make an imagination effort and infer why is that. $\endgroup$ Oct 7 at 11:06
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    $\begingroup$ @NotThatGuy A normal bird has the flying muscles strongly anchored by their sternum and with the rib cage underneath. Without the same support on the now-the-flying-side (also containg the uterus), there's going to be quite a churning of the organs underneath with every flap of the wings. And this is a problem for the eggs. $\endgroup$ Oct 7 at 12:39
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    $\begingroup$ Once the eggs are sealed up and hard, there is no benefit in carrying them any longer, they should be laid as soon as possible. Per scienceline.ucsb.edu/getkey.php?key=3261 it takes only 20 hours for the chicken egg to harden, so the risk of breaking hard eggshells could be only a temporary problem. $\endgroup$
    – Dave X
    Oct 7 at 14:28
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    $\begingroup$ Am I the only one that understands, from the title of the question, that the guy is asking about flying eggs with legs ? $\endgroup$ Oct 7 at 15:52
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The uterus does not participate in the mechanical stresses of movement.

It is not invited to that party and it does not want to go. A uterus has uterus things to worry about. It no more participates in skeletal movements than any other smooth muscle structure. Stresses of movement are distributed about the skeleton and core muscles.

As regards the contents of the pelvis including the uterus, the pelvis is a formidable bone in normal birds (and pretty much all vertebrates) and would only be more substantial in your leg fliers. The anterior extent of the pelvis is the pubis which provides some support on that side - both to the uterus / oviduct as well as digestive tract structures.


When your creature needs to use its reproductive tract or digestive tract to get leverage to move around it is doing something wrong. That goes for you too. If you find yourself doing that, quit. Or take a video of you doing it and post it, then quit.

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  • $\begingroup$ "the pelvis is a formidable bone in normal birds" and you have one insertion point for those muscles. Question is: where's the other one? Because it's very likely the two wing will need to beat independently one of the other, maneuvers and what not. $\endgroup$ Oct 7 at 11:08
  • $\begingroup$ +1. This needed upvoting. $\endgroup$ Oct 7 at 14:04
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Soft Shells

Without being much of an engineer it seems like the best way to handle this from the bird's perspective would be to lay a soft-shelled egg that hardens upon exposure to the air. The softer shell would be able to flex in the uterus until it was time to lay it, so the flight action wouldn't be able to crack it. Depending on the mechanical stresses involved/how many eggs are laid at once etc the birds might evolve to lay eggs somewhat smaller for their size than normal, or fewer eggs, to ensure maximum room in the uterus.

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    $\begingroup$ I wouldn't (necessarily) even expect them to harden after hatching. Plenty of reptiles already lay 'soft' eggs with a leather-y type of shell $\endgroup$
    – TCooper
    Oct 6 at 21:06
  • $\begingroup$ Ok, an experiment for you: rub you fingers on the same place for about two hours, at about 0.75Hz. I bet you'll get the idea that even soft shell can scar. (I was going to suggest penetrative sex for the duration, but... I abstained) $\endgroup$ Oct 7 at 11:16
  • $\begingroup$ OP could also write that the rhythm of flexing the egg helps teach baby birds to fly in utero. $\endgroup$ Oct 7 at 12:34
  • $\begingroup$ @AdrianColomitchi Animals that fly with their legs - the legs don't flap like the arms do, they're there mainly for added lift and attitude control, kind of like ailerons or elevators on a plane. $\endgroup$ Oct 7 at 13:56
  • $\begingroup$ @DarrelHoffman you followed the link to his prev question for the context? How about the description of "What would be their wings have no use in flight... This gives them more room to flap their wing-legs.." $\endgroup$ Oct 7 at 14:10
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I'm not convinced it would be a problem.

Eggs crack easily to impacts, but have incredible compressive strength. So it's trivially easy to break one with a knife or fork or edge of pan, but just try squeezing one to death. You may not be able to do it, and if you do it will take a lot more effort than you'd think at first. This was a bit of a meme in the 20-teens.

https://www.youtube.com/watch?v=2J6pM0y1fYE&ab_channel=GinaSu

I was also going to suggest the eggs could be soft-shelled and harden on contact with air, but Dario Quint posted that answer before I was even halfway through writing this. That could work too I guess.

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  • $\begingroup$ "Eggs crack easily to impacts, but have incredible compressive strength. " Move them around by muscle pressure and the shells will ground between themselves to sand. Or the more mineralized one scar the soft tissue of the other. $\endgroup$ Oct 7 at 11:11
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    $\begingroup$ @AdrianColomitchi That relies on the assumption that the mother has multiple eggs at once in the same chamber with no barriers between. If only one egg can grow at a time, or they are divided by soft tissue, that's not a problem. At this point, you may as well ask, "How do eggs of real animals not get destroyed in utero?" $\endgroup$ Oct 7 at 18:07
  • $\begingroup$ And yet people without much muscle mass can break eggs with their arms. digg.com/2021/woman-cracks-egg-with-arm $\endgroup$ Oct 7 at 22:37
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Would the suggestion of the female going into a period where she does not fly during breeding season prior to the egg being laid violate the "Not fly at all" rule? This would see her enter a dormant or hibernation like period proceeded by a period of aggressive eating to build up fat stores for the time she would be unable to find food. Alternatively, the male mate would be tasked with gathering food for the mother and himself. When the egg is laid, the mother would be free to resume gathering food and may do so in shifts with the father OR leave the father to incubate the egg himself (the latter behavior is famously observed in Emperor Penguins. The female is going to almost exclusively replenish her food reserves, having not eaten once the egg began developing and only resuming once it is laid. The former behavior is observed in all other penguin species, where the male and female take shifts incubating the egg).

It should be pointed out that mating for most animals is either done seasonally, or opportunistically (when environment conditions are correct). Humans are one of the few species of animals that are continuous breeders (Animals that are capable of breeding at any time of the year) and the only non-human species that are capable of this are great apes, all closely linked to humans in an evolution standpoint. It would not be out of the realm of possibilities that your bird would cease strenuously flying just prior to the species mating season, to allow for the egg to develop until it is laid.

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  • $\begingroup$ " Humans are one of the few species of animals that are continuous breeders" Freeing the wings to be used as hands suggests LiveInAmbeR had in mind pushing them towards intelligence and technology. Which will appear only in highly social species (you need the division of labor). As the level of intelligence grows, the duration of the childhood is bound to increase (the longer the kids learn, the more experience can be passed between generations, the better for the species). I suspect that those birds will get to show the same sexual/breeding pattern as humans do, just give it 1000+ generations $\endgroup$ Oct 7 at 11:34
  • $\begingroup$ Hornbills do something like this. The female is sealed into the nest and the male feeds her through a tiny hole. youtube.com/watch?v=5xQR-8EVuX8 $\endgroup$
    – JDB
    Oct 7 at 13:47
  • $\begingroup$ @Adrian Colomitchi Calm down Darwin they’re just birds. The little bird hands was a creative decision to give them something to work with now that their legs are used purely for locomotion. $\endgroup$ Oct 9 at 1:14
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enter image description here

  1. Make the sternum prolong after the joints of the feet-would-be-wings - because you will need the flight muscle to connect to it and still let enough room for the guts, kidneys, cloaca, etc - but don't let it stick directly to the "pelvic basin" bones, because you need the space to pass the eggs

  2. take the wishbone from the front an make it connect with the pointy tip of the V on the end of long sternum and arms of the V on the pelvic girdle. Sorta collar bones, but at the other end. Just make then thick enough to resist the compression from the flight muscles as they beat down and pull on the sternum

  3. you can lose some height from the sternum in front, as the breast muscles are no longer involved in flight, and make it quite heavy on the other end, where the flight muscles insert.

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  • $\begingroup$ I'd suggest moving the legs further apart, then more forward into the rib cage area. Without the wings for stability up top, you need to move the center of mass lower. You need to have the center of mass and the center of lift near each other for stable flight. aviation.stackexchange.com/questions/32271/… and aviation.stackexchange.com/questions/35162/… $\endgroup$ Oct 7 at 22:42
  • $\begingroup$ @computercarguy stable flight is not a requirement. Bats manage to do without quite wonderfully, in fact maneuverability is an advantage for insectivore diets even more so for flying in caves. $\endgroup$ Oct 8 at 18:08
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Eggs don't take long to form (chicken eggs take about 24 hours). Could they simply not fly for 24 hours while they're making an egg and then go back to business as usual afterwards?

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Does this creature have to lay eggs?

Could it be a marsupial with feathers? Having a reproductive process similar to kangaroos and a similarly placed pouch to carry the infant.

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  • $\begingroup$ Oy, matey. You let the kangaroos alone or I'll send a drop bear to your house, with a jar of Vegemite :) $\endgroup$ Oct 7 at 14:14

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