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In the world I am designing, instead of mammals, birds are the dominant species. In this world, replacing the niche of deer gazelle and cattle are a group of animals known as Quadrabirds. These are similar to flightless birds but with the their wings turning into a type of walking limb, hence their namesake.

What I am wondering is how such an animal would evolve? My current thoughts are related to the fingered bird, The Hoatzin.

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  • $\begingroup$ ever though about griffins without wings? $\endgroup$ – Charon Aug 14 '16 at 9:58
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I think the Hoatzin is a good starting point. The young have claws on their wings that allow for climbing. These claws are lost in adulthood, so the first evolutionary step is for a Hoatzin to evolve neoteny, retaining the claws and becoming secondarily flightless. It does this to save on developing wings and flight muscles, and to exploit the niche taken by monkeys.

Larger Hoatzinids become primarily land based. They climb trees as needed but rely on their claws for defence. They eat leaves and fruit.

However climate change results in a shrinkage in forests and so some evolve to live on the new grasslands that are taking their place. Here there are predators like eagles, so extra bulk is an advantage. The obvious food source is the grasses so that is what they eat. Their feathers are reduced to a thin covering (except for tufts that the males sport during the rut) and their claws are mostly used for digging for roots and the annual rut. They swallow stones to help break down grasses in their stomachs.

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Your quadrabirds don't have to loose flight to have a walking fore-limb. Another answer already mentions that some groups of pterosaurs folded their wings when on the ground and used them for walking. There is an excellent article about this terrestrial locomotion at pterosaur.net, which includes some nice sketches. (I've said it before on wb.se. Are you folks not getting this yet? Modern literature needs more quetzalcoatlus.)

There is also a living species showing an example of convergent adaptation, the New Zealand Lesser Short Tailed Bat, (Mystacina tuberculata). In this case, it is not hard to find videos of their walking pattern, so you can actually study the mechanics of it while designing the quadrabird.

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Well the short answer is that they probably wouldn't, bird forelimbs aren't designed to support a load in that way. Not to mention that, for all intents and purposes, simply flying away from your predators is better than running from them in most circumstances.

But given that this is fiction, we don't need rules; check out the Azhdarchids. There's a lot of evidence to suggest the sort of quadrupedal stance your Quadrabirds would have.

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    $\begingroup$ Good call. The Azhdarchids make sense as the basis for something like Quadrabirds. $\endgroup$ – a4android Aug 14 '16 at 9:09
  • $\begingroup$ flying giraffes $\endgroup$ – Charon Aug 14 '16 at 10:00
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Theropods

Take a look at the Theropods. They are the precursors to modern birds. Some of them were plant eaters which puts them near enough in the same niche as deer. With a little imagination, no mass extinction events, and maybe a much heavier animal, its easy to see how they could slowly adapt to walking on four legs.

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In a previous answer I suggested ways to evolve avian wings back into useful limbs for another purpose. One idea I like is that the wings evolve into legs. If you don’t need the manipulation of hands, getting 4 walking limbs with hooves in front should not be a problem.

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