Plausible evolutionary path from modern birds to "dragons"

Question

I'm interested in how you could get a creature that is broadly "dragon-like" to evolve from modern-day birds. Considering that some extinct early birds or proto-birds looked pretty reptilian, I think it's not an impossible route.

Parameters:

Minimum definition of "dragon" for the purposes of this question

• Capable of flight
• Large enough to be terrifying and potentially capable of picking up an adult human (likely with talons) for at least a short distance. This would have to be a huge bird, at least as large as the extinct Haast's eagle from New Zealand (up to 10 ft wingspan) or even as large as some prehistoric birds (for reference, the heaviest flying bird ever, Argentavis, weighed in at 72 kg with a wingspan up to 21 ft)
• Head doesn't have to necessarily be serpentine or reptilian, but it should have a distinctly extended head and neck (like a vulture) with at least a face that is mostly or entirely featherless
• teeth would be great if possible, although no modern birds retained teeth

I know it kind of just sounds like I'm describing a giant vulture, but I'd like to know if it would be plausible to push it more into dragon territory and what kind of evolutionary pressures would push birds into developing those adaptations. For the purposes of this question, assume we're starting with modern Earth birds, but they don't necessarily have to be located on Earth (could be a terraformed exoplanet, for instance).

Answer 1

Arid climate

Which, funnily enough, could push a vulture closer to a stereotypical European dragon of legend in every way.

There's a few main factors at play here:

1. High temperatures
2. Water scarcity
3. Foods available

These three conditions would favor the following traits, respectively:

• Loss of feathers, as heat retention is no longer needed.
• Development of scales, to conserve water (with the added benefit of protection from the hostile terrain). The feathers would likely degenerate into these, rather than disappearing entirely before.
• Redevelopment of teeth. This one's a little more iffy, because the beaks of vultures are already optimized for tearing meat, but genetic drift could cause them to reappear.

Essentially, the vultures are evolving back into lizards, keeping their ability to fly.

Now, the loss of feathers could pose a problem to the "flight" portion of your requirements. I am not sure how much replacing feathers with scales would affect the flight capabilities of your birds, but unless the atmosphere is super dense, or gravity is unusually low, they may end up grounded for a time in their evolutionary period, before developing an alternative, such as webbed wings.

Answer 2

The problem with evolving from birds is that birds usually have hollow bones, making them frail, as such, they are unable to fight similarly-sized prey, making them usually scavengers. If you want to super-size them, they would need to be light and very brittle, making them not really pose a huge threat to people. Making them partially featherless only adds more problems, so I suggest starting from lizards.

If your dragons are evolving from lizards, there are a couple of changes to the dragon that would increase plausibility. Being a wyvern (2-legged) as opposed to a traditional dragon would be more probable since it would only require a membrane to fuse with their preexisting front limbs, as opposed to sprouting entirely new appendages. On that note, you should strongly consider your dragons to be only capable of soaring as opposed to flying (the distinction being the ability to generate one's own lift). This isn't as limiting as might be expected due to the preponderance of thermals, and other air temperature disparities. This limited ability is also consonant with the evolutionary route since it would be very improbable to develop hollow bones before a creature is capable of flight. Therefore your wings will likely be unable to compensate for the weight and produce lift. If you want them to have greater maneuverability (comparable to a glider aircraft)in the air, you could give them rear wings, like the Changyuraptor.

Another consideration is their size; If they are large, they need more energy and sustenance. A small lizards diet is inadequate, so it too must change. The ancestor lizard's prey should also have adapted (gotten bigger, stronger, etc) or died out (killing off small lizards due to starvation while allowing larger ones a chance at finding new prey unfeasible to the rest, causing the large mutation to be more common). Eventually, humans might become feasible prey. You should also know that once they become a dominant species their evolution will stop advancing as rapidly. In this scenario, teeth are especially viable since adequately large prey would require breaking into pieces.

To summarize my design suggestions:

• Wings like a pterodactyl (important if feathers are not in play)
• Wings on front and back (I'm less sure about this, but it makes them have greater air supremacy)
• No front limbs that don't have wings(greatly improves plausibility)
• Only capable of gliding
• Very diverse food chain or pack-hunting(for size)
• Omnivore but primarily meat-eating (greater energy to match consumption)

Although it's not quite the bird-dragon you wanted, its more plausible, and it's not just a stereotypical dragon either. I also didn't mention it much, but if you make them pack hunt (~human size - more plausible), you could have a more unique creature. Imagine how much more terrifying a pack of vicious, nearly man-sized lizards descending and devouring a person is than a big dragon-bird that takes one bite and is done with it.

Answer 3

This is probably not what your wanting, but if your dragon-birds were originally selectively bred starting from carnivorous birds for their size, strength, featherless head (unless they started off with vultures or similar birds),(and possibly teeth) and a longer neck (probably not all at the same time) to taxi people around faster than cars do. That would open up the possibility of a group of them to escape and breed to be more aggressive and would already have the strength to carry people off, which would give you everything except the teeth. Giant escaped taxi birds turned aggressive probably isn't intended but it's a possibility.