Harpies, AKA 'the flying ones', are currently the youngest strain of the primal mutation, at least in terms of officially recognized documentation.

They occupy a unique position within the many variations of the curse, as though there are countless unverified reports of it cropping up in the other strains, Harpies are most well known as the only strain capable of assuming avian DNA. This does not mean they are exclusively limited to birds, as some subjects have displayed traits of bat or even flying squirrel, but the vast majority of all known Harpies have been birds, and the vast majority of all known mutants with bird-like traits, have been confirmed to be Harpies. The entire avian side of the kingdom animalia and the Harpy strain, are intrinsically linked.

This is due in part to the main biological feature of the Harpy strain: Flight. Originally the strain was assumed to be a drastic deformation that mutants of the Warrior strain would undergo, due to the incredible similarity between the two, but after numerous studies of Harpy cadavers and a noticeable lack of evidence depicting a Warrior suddenly sprouting wings, it has since been discovered that there is a significant difference between the mutations and how they manifest within the human body, enough to confirm that the wings were not a spontaneous mutation, but rather a newly gestated strain of the curse itself, marking it down as the Harpies we know today.
And it is these wings that stand out as the Harpy strain's most significant trait, as their size and scale are often broad enough to efficiently catch into natural wind currents, and provide lift to the entire body, much like the flight-capable animals they are based upon. There are even records of these wings increasing or decreasing in size depending on a Harpy's situation, either shrinking into compact forms to fit in smaller spaces, or stretching out into massive scale to provide better flight strength. It is estimated that this expanding and contracting of a Harpy's wings is either an instinctual, claustrophobic habit, or perhaps a remnant of the mutant's mind still able to recognize the concept of space-efficiency, as the process appears almost effortless for a Harpy, save a moment of focus and supposed discomfort.
And these wing mutations themselves can manifest in various ways, most often overtaking the infected's arms and outright replacing them, either consuming their hands and removing them from the body, leaving the appendages be and sprouting along the forearms, or a middle ground were the hands are warped into bizarre, swollen hybrids of feathered "wing hands". Just as often though, a Harpy can retain their original arms, and instead sprout the wings across their upper or lower backs as entirely new limbs, granting them the fortune of gaining flight, without the sacrifice of their human dexterity. Some far less fortunate Harpy specimens meanwhile, may even have their wings sprout in...asymmetrical configurations. And whether or not a Harpy with a single wing out of its back and an arm warped into the other is capable of functioning, let alone flying, within the chaos of a mutant raid, seems purely dependant on how well the Harpy's original mind can cope with it. Usually, it's not a pleasant sight.

And though the arm wing variations greatly sabotage a Harpy's ability to wield tools and weapons like their peers, more often then not mutants of the Harpy strain, regardless of their wing positions, may not even care to use their hands. That is because their bodies are similarly adjusted to sport large, articulated talons, complete with opposable toes at the rear ends to allow grasping functionality. And Harpy mutants seem to be incredibly well-adapted to using these talons almost like a replacement, or second set of hands, allowing them to perch on seemingly inaccessible terrain, carry various tools or cargo, or most concerningly: Capture prey without even landing.
Which brings up the major threat of the Harpy strain. With our ever expanding war against nature, humanity has had to deal with airborne threats for quite some time, yet outside of vague records of pre-reconstruction history, there has never been a human-made, organised militia, that housed flight-capable weaponry or infantry within their ranks. Or at least that was, until the gestation of the Harpy strain.
Whilst not as durable or threatening as a Hunter or Brute strain, and usually nowhere near as intelligent as a Chieftain, the mere capability of just ascending up walls, barricades, buildings, trench lines, and most other fortifications, grants Harpies a devastating advantage against human defence systems. Without AA cannons or ceilings to stop them, a Harpy could just ignore any obstacle a settlement can plant in their way, allowing them easily target vital backlines, harass critical systems, outmanoeuvre slower, heavier equipment, or even bypass security measures entirely, to attack civilian non-combatants. This last feature is a Harpy's most horrifying trick, as their flight allows them to quickly intercept retreating bystanders regardless of their position, and either snatch them off their feet to devour, or worse still, scrape or bite at them to transfer the infection. In the eyes of the public, this has marked Harpies as ideal "infector" units, as their manoeuvrability allows the curse to spread and fester in areas that'd otherwise be completely safe from raiding hordes, at rate far faster than usual. Many civilian bunkers have been found to have been tragically overrun or devoured by unchecked infections, often purely thanks to a Harpy swooping down into a panicking crowd, and getting a lucky swipe in.
On top of this, their ability to reach incredibly high elevation, combined with what could only be a drastically heightened, raptor-like sense of sight, allows the Harpy strain to comfortably seat itself as a perfect "Scouting role", scanning every defence and direction and point of interest within any settlement a horde may have interest in, without the settlement ever even noticing.

Thus, though not massive threats in a direct confrontation, this incredible, inhuman utility has quickly labelled Harpy strain mutants as massive priorities to be dealt with as soon as possible.

Fortunately, this is far from impossible to do. Whilst airborne, a Harpy is usually incredibly difficult to target without A.I assistance, especially with slower bolt or thrown weapons, but surprisingly they don't seem very well adept at taking advantage of such. As stated before, it's quite easy to see Harpies as just flying Warriors, for Harpies have not only had their intellects very poorly preserved by their mutations, but seem to boast an even higher sense of aggression an overconfidence than their detrimentally brave Warrior kin. This means that, once they see a threat, a Harpy will not exactly remain airborne for long.
Some do seem to recognize the benefit of remaining at range, preferring to fly out of reach and-if they have enough limbs for it-pelt their opponents with ranged weaponry. But most however, either try to fly far too close, or even just land right in front of their enemies to engage in close quarters. And with their limited capability to use tools, slightly more brittle construction, and instinct infested intellects that make them almost indistinguishable from actual birds, close quarters are exactly the worst spot a Harpy could find itself in. And yet, more often than not, a Harpy will think its best to make the least beneficial choices in a one-on-one conflict.
As such, this makes Harpy mutants more annoying to deal with, rather than threatening. Not only are their fighting patterns very predictable or undignified, often closer to frantic flailing then actual efforts to harm, they also tend to not be very quiet when in combat. Many Harpies seem to suffer from so much pent up aggression, that they engage in battle whilst screeching and cawing with almost immature ferocity, and yet outside of very specific mutations of the strain, the noise does nothing but irritate. The best method to combat them is just to keep distance of your own, have a weapon stronger than a bow and arrow, and maintain focus in spite of their fury, as a Harpy's excessive eagerness for combat may or may not be used as a diversion of attention.

But above all else, when dealing with a Harpy of a mutant horde, you must make sure it is dead. Because due to the highly contagious nature of Primal mutation, a raiding horde has not been fully dispatched until every single member has perished. And the case of this strain especially, thanks to their ability to breach the unbreachable, as long as a Harpy remains flying nearby, a settlement is not clear of danger. You must ensure that nothing remains of the curse at all, no stragglers on the streets, no infected limb unchecked, no exceptions spared. Nothing, of a horde, can remain.

For the cruelty of Nature banks on us missing a spot, no matter how small.

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Okay so this bird was supposed to be a Shrike specifically, but that species is so niche and obscure that obviously FA didn't have an option to put it in species lable. I'm annoyed cause the others were but not this one. My OCD is furious.