Evolving an intelligent avian race with a human-like appearance

Question

Having read this excellent question and its answers, I've tried to come up with an evolutionary history for a sapient, avian race that populates part of a world I'm building. I'd like feedback as to whether what I've come up with so far makes sense, and help filling in a few gaps, especially in regards to how the race could develop humanoid morphology. Currently the race isn't coming out looking human enough for my liking (or my storytelling purposes).

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Imagine a species of large, tropical birds (approximately a mixture of macaws and hawks). They are not predators, but their lifestyle involves "hunting" by soaring over the forest canopy and using their keen eyesight to pick out specific fruit and nut trees. They are largely solitary, with mating pairs meeting up once a year and otherwise going their separate ways, but not territorial.

Then, catastrophe strikes, and an environmental disaster ruins their rainforest home (it's an alien invasion, but that's not important to this part of the story). The birds who survive head west across an enormous desert dotted with oases. Through the course of this exodus, the once solitary birds gradually form flocks, since they keep congregating at oases. They also find that there is safety in numbers, both due to the desert being filled with strange new predators and due to multiple pairs of eyes being better at spotting food and water. Pairs begin spending their whole adult lives together, since it's much harder to find each other in an environment their navigational instincts aren't attuned to. To feed themselves between oases, they expand their diet to include protein-rich insects (this world's equivalent of locusts).

Eventually, they spread to reach the other side of the desert. Here the land is more barren, with steep stone cliffs and spires. It rains more often here, however, and there a variety of shrubs and trees with highly nutritious seeds, plus plenty of juicy insects. Some of the birds settle in rocky caves high above the ground and begin to create social clans composed of family units.

The local flora evolved without any major grazers, and the birds are forced to become very careful with their feeding habits in order to sustain their food sources. Knowledge is passed down through family units and is improved upon by subsequent generations. Before long, they're storing seeds in their caves in preparation for dry seasons, and then deliberately planting seeds to replenish the population. Primitive agriculture develops, and stone tools emerge. It's all uphill from there.

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That's what I've come up with so far, but there's one point in particular that confounds me. Given this evolutionary history, how, when and why would this race evolve to look vaguely human? Ideally, I'd like them to wind up with the following phenotype:

• They're comparable in size to humans, maybe a bit thinner.

• They still have digitgrade legs and talon-like feet. They can't fly anymore but still have fairly porous bones and weigh less than humans.

• They have beaks, but they do not protrude far and their faces are overall more flat and human-like than most birds.

• Large eyes, Voldemort-like noses that merge into the upper "lip" of their beaks.

• They have feathers instead of hair, with full feathers on their head and back of their neck and tiny, down-like feathers in place of body hair.

• They have manipulating fingers with opposable thumbs, and no wings.

Most of these I feel comfortable hand-waving, but the first and last point stand out to me as being especially troublesome. The two things I just can't figure out are the increase in size and the replacement of wings with arms.

I think arms that could be used for climbing would be more useful than wings in an environment combining sheer cliffs with winding caves, especially when agriculture and tool making enter the picture. But I don't see how wings could realistically evolve back into arms. All the bones are still there:

But the bones of the hand and finger appear fused and embedded within the blade of the wing. I can't think of any transition state between a full wing and an extremity with fingers that makes any sense. Note that their arms and hands don't have to have the exact same structure as ours; one idea I had involved making the metacarpals a third section of the arm (with the bones of the "wrist" joint forming a "second elbow" and having the phalanges extend directly from the end of the arm without having a palm).

TL;DR: Why would birds evolve to be as large as humans, and how could wings possibly evolve into arms with manipulating fingers?

Edit:

I was assuming the hands would have to stem from the bones of the wing, but see the follow up for a different approach.

Answer 1

They don't, at least, not the way you want

Absolutely everything about bird is designed to create a streamlined animal, from their beak, to their legs, to their loss of fingers. Making a bird look more like a human requires going back, far back. In fact, it requires going back to early bird evolution.

The main problem here is that what we consider birds are so specialized that we have our image of bird generalized. When I say think of a mammal, people will think of many different shapes and designs, not with birds though. We must consider that your 'birdman' will be different from regular birds in every way. They will have to separate from normal birds extremely early in their evolution, becoming a multipurpose glider.

As they evolve they will keep one talon finger to help them climb, only one so that all the other digits can make the rest of the wing, I'll explain why there cannot be two or three later. This single finger is likely to evolve from the Alula. Their wings and tail feathers let them glide down onto prey (I suggest redwood forests for their evolutionary enviroment) and yet their talons and single finger lets them climb up trees to nest and glide again. The reason from their massive size will likely be due to fact that whatever they 'prey' on (or more likely slowly eat alive) will be massive, like a sauropod or Giant Mammoth.

This great size also is what limits the number of digits, If we did an even split of two fingers for the hand (plus a thumb) and two finger for their wings, it is likely that their wing will not be strong enough to even let them glide. This is based on bat evolution, but the point still applies.

Keep in mind this answer is made to help you with human sized bird that have wings with fingers.

Answer 2

There is no reason why these flightless birds that evolved sapience to need hands and opposable thumbs. One of the victims of thalidomide, who was born without arms, his hands emerged from his shoulders, became a radio DJ and was able to manipulate vinyl records and anything else he wanted to using his feet. He slipped off his shoes or sandals and grasped objects between his toes, while in a sitting position. It's surprising how prehensile toes can be, and very effective at picking things up.

From this, it's no great stretch for bird aliens to further develop their feet to take the place of the manipulation primates like us do with our hands. Most bird species do something akin to this already.

Just allow their feet to do more than walking. Admittedly it would be a bit of a balancing act, but certainly not beyond the bounds of possibility. So there's no need for them to re-evolve hands from the genetic dead-end of having wings, even if they had become flightless.

Answer 3

On Earth flightless birds never recover “hands”. Has the code been trashed since it was not being expressed? Let me speculate a few ways out of that.

First, what if the wings were not like Earthly avian wings, but more like bats or pterosaurs in which the hand bones remain. Then they could evolve back into a grasping role.

Second, what if the code wasn’t lost? There are mutations where the wrong limbs or the wrong body segment grows. A mutation could make the animal develop the upper limbs using the same plan as the lower, re-introducing elbows and grasping talons.

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Let me add to a4android’s answer and suggest that the talons become fulltime hands (as he explains in another answer) and the wings evolve into legs!

Without recovering the complexity of a hand, the upper limb can still become a hoof-like foot. First the wings are used to steady a d stabilize the body while using one or the other foot as a hand, and things evolve from there. The manus could become hinged again without having to recover individual ankle and foot bones, or it could be used as the foot with the nest joint being ankle.