How plausible, or implausible, would it be for a lineage of birds' feathers to evolve into sharp, hardened plate-like scale armor (and/or blades), comparable to that of pangolins? And if it is plausible, which ecological niches would these birds be best suited to fill?
There is no reason it couldn't evolve, but what are its limitations and niche?
The bird would almost certainly be flightless. Flight is precluded because in order for the armor to be effective it has to be strong, and dense. Stronger armor means heavier scales, and heavier scales means flight will be increasingly difficult to accomodate. This is kind of a one or the other scenario. In order for the scales to effective the flight part of this animal has to be sacrificed.
If we aim for a middle ground of keeping both adaptions then neither adaption (flight, or armor) will be effective. In the scenario of having both adaptions, evolutionary pressure would probably favor reverting back to flight since it is a smaller change. See Occam's razor.
This bird's wings would probably be vestigial and destined to disappear. This is because the wings would serve no practical purpose anymore, except perhaps as implements with which to manipulate objects.
Since this is a flightless armored bird, the most obvious niche for it to fill is that of other flightless birds like the chicken, or Cassowary. However something that has evolved the necessity for armor plating must also have an overwhelming threat from predators that would make the other two types of morphology unfeasible. Otherwise the adaption would never evolve because presumably these other two morphologies are easier to reach. All these factors will shape its niche.
I surmise that you will find this kind of bird somewhere isolated, where predators are highly prevalent. It will occupy a space that other birds cant because they are killed too often for a population to survive in that place. However in this niche the armored plates of our new breed of bird will protect it from these threats. In this space, the armored bird may be the only bird around, and less armored variants struggle; otherwise the evolutionary pressure will be to lose the armor. One suitable habitat is an island infested with cats and leopards, and snakes, and bird eating spiders, and so on. Heavy armor is a necessity, rather than a luxury.
This species can be endemic to that island, and so be found no where else on the planet.
Isolated islands are a good contender for exotic adaptions like armored birds for the following reasons. The space is often limited; Competition is very high; and escaping unfavorable circumstances by looking for new habitats is not always possible. These limitations can lead to an Evolutionary Arms Race, specifically the Red Queen's Race; which is one possible cause of a highly specialized adaption like armor.
In an Evolutionary Arms Race a predator evolves to hunt its prey more effectively, and its prey evolves to survive more effectively in response. As a basic example, a predator that evolves to be faster in order to catch its prey may cause the prey, in order to survive, to evolve to be even faster. Now that the prey is faster, the predator now again evolves to be (even) faster in order to catch this faster prey. When this happens the prey evolves again to be even faster so it can escape, and so on. This goes on and on in a loop until eventually both the predator and prey are absurdly and implausibly fast; far beyond what we would expect under normal circumstances. In this way an Evolutionary Arms Race can lead to wildly exaggerated traits like heavy armor and other defense mechanisms. One real world example of an Evolutionary Arms race is this toxic newt.
An isolated island is a good contender for this situation because resources are limited. The only prey available to this island's predators may be this one armored bird. So both species may get locked into a race that neither can escape. For example leopards on our island may get better at cracking armor, and our armored bird may evolve thicker and thicker armor in response, or get better at killing predators.
This is one situation that could lead to pangolin armored birds.
RonJohn makes an excellent point about the evolution of natural armors in the comments, that I would like to highlight here:
Note … that feathers just can't evolve into armored scales unless there's an evolutionary path. ... there is a reasonable path from feathers -- made of keratin -- to armor made of keratin just like the pangolin. You just need more of the same thing packed/woven closer together.
Tyler S. Loeper has an excellent answer about the "how" of adaptations, so I will focus on the "why".
The need for protection implies some sort of threat, but it need not be the threat of predation against the birds themselves, since, as pointed out, readapting for flight is far easier and more efficient (flying provides access to safety, other food sources and other ecological niches to exploit).
I will suggest armouring is appropriate for large, ground dwelling predatory birds, like the Phorusrhacids. The "Terror Birds" evolved their large size and ground speed to occupy the niches once filled by theropod dinosaurs, (which makes sense, since they are actually descendants). In a timeline where South America does not join with North America, both the Phorusrhacids and their prey will evolve in a mutual arms race. Some herbivores will eventually develop horns or antlers, and defensive strategies like moving in large herds, becoming very aggressive or forming circles around the young and vulnerable when threatened. Predatory birds will adapt to overcome this, and one or more species may well evolve feathers into armour to protect them when going in close to their prey.
Phorusrhacid on the hunt
Feathers are thought to be derived from modified scales in some of the ancient dinosaurs from which birds are derived.
So having feathers that turn into scales would be the same path in a different direction. However this is entirely possible. If you consider that nails are modified hair, it's not unthinkable that the feather's implementation plan could be altered to give a more compact structure.
Such a bird would have no ability to fly, though. I imagine they would be some sort of reinforced kiwi, rummaging the ground in forests where they can easily find shelter and find nutrition. Basically a two legged pangolin...
Maybe not all you'd hoped for (other answers seem to assume you want a bird armored from beak to talons,) but I can see a bird developing armor on the leading edge of its wing, or at least a portion of the leading edge of the wing.
There are already species of bird which like to use their wings as bats (looking at you, Canada
devils Geese.) Furthermore, many types of predatory bird species rely on a powerful diving assault as part of their hunting strategy (like a Peregrine falcon.) It's quite feasible that a bird which likes to divebomb or bash would develop some hardened scales/armor in a location where they can be used to concentrate the force of a crushing blow.
I don't know how seriously this would change their aerodynamics... the extra weight would definitely be a huge penalty on their speed and maneuverability in the air, but I imagine that a small plate of armor wouldn't completely prevent them from flying.
Alternatively, the patch of armor could be a trait only found on males, who use this armored section in their fights with other males during mating season.
So I just did some research and calculations, and I've found that an eagle with scales similar in thickness to a carp's scales would still be able to fly efficiently. I'm not going to link the six sites it took me to gather all the necessary info, nut the calculations come out to something like the weight of scales would equal 1/4 the eagles maximum carrying weight, assuming a larger than average eagle in terms of surface area, with a lower than average carrying weight.
Luckily, feathers are basically just very very very thin armor, with the quill of the feather being hard enough to be useful. If we imagine a bird whose quills become thicker and the rest of the feather hardens, we can get scales with minimal impact to the structure of the bird.
Now this bird has a ton of sharp hardened feathers, just imagine how it would rip and shred non armored birds or other non armored creatures just by flapping or beating it with its wings. Combined with sharp talons and beak, there isn't a single spot on this bird you'd want to be on the wrong end of.
But why would an eagle sacrifice a portion of its range and lifting ability in order to gain scales? Well, I imagine a new very aggressive species of bird which form small flocks and steal prey from predators by driving them off. If the eagle tries to take the food with it, they fly up and attack it from behind. But with its large wings it can beat back these birds and eat the food where it is. However, sitting still is a disadvantage for birds, so armor is a relatively cheap way to both defend its back from the new birds and also mitigate ground threats. Plus, armor converts its body into an additional weapon, its wings becoming not just powerful but also dangerous.
But say, if it cant carry as much wait for as long, isn't that a disadvantage? Well sure, but eagles are amazing gliders. It simply carries its prey or stick high into a tree, and then rests on a branch. When it takes off it can fly up a bit higher and then slowly glide down, basically falling with the weight but across a large horizontal distance. If it needs to rest it can always perch on the top of a tree.