Bird-like wings, which can be arm wings or separate limbs on the Harpy's back. Minimum size allows for gliding, but larger wings may allow them to fly.
Talons and bird-like hands and/or feet, per player choice, as well as sharper teeth. This is all of them, not including just the canines. No other special features come with this change though.
Pointed ears. May also include feathers, depending on the Harpy.
Feathers covering a large potion of the Harpy's body, focused mostly on their lower body but around the back/shoulders near wings. This includes a feathered tail, used for steering, which can be any length of the player's choosing.
Two necessities that assist in flight, in two different ways: hollow bones, as per the norm for birds, and small air sacs in addition to their natural lungs. A bird's respiratory system is far more efficient than that of a mammal's, and Harpies - despite being part mammal - inherit some of this efficiency. Thus, they have a greater lung capacity and can travel at higher altitudes without assistance.
A nesting instinct. Harpies prefer being higher up as opposed to being grounded, as this may leave them more exposed and vulnerable to threats. As such, they perch on buildings and in trees more often than not, and even if they're indoors it's likely that they'll find a way to move their bed into a higher location. Whether this is a nook in a wall or an overhang depends on preference. Nesting will vary based on breed of bird, but also takes the human side into account.
A more feral looking form during the full moons, which can also be triggered during high emotion situations. Feather growth increases and spreads, wings increase slightly in size, hair/feathers get more wild, and teeth and claws may elongate and become sharper. As notoriously aggressive Monsters when agitated, this aggression may increase - but regardless, a more feral mindset will also kick in for Harpies that have a harder time with their less human instincts, the ones that fight the change.
OPTIONAL CHANGES
A fully feathery, bird-like lower body with wider hips, digitigrade legs, and fully functional grasping claws that vary in appearance based on the bird in question. Legs may have additional feathers on the lower portion depending on the bird that the Harpy is based on.
Additional digits on wings that allow for grabbing, especially in those with arm-wings.
Additional muscle growth around the chest, shoulders, and/or back to further support their wings. This may manifest as making the Harpy in question look like they have a second set of pectorals, for example.
Hair and/or skin changing to reflect the coloration/markings of their bird in question. This can be a complete or partial change, per player choice. Hair may also include additional feathers in it.
Torpor in select Harpies whose species of bird may enter a torpid state during cold weather and when food is scarce. While not an actual hibernation, Harpies may find themselves incredibly lethargic and wanting to sleep as their body temperatures, respiratory functions, heartbeat, and metabolic rate lower. Extra downy feather growth for insulation in colder weather may help offset this.
Harpies who have completed most of their changes can choose to shift into a full bird form. This shift is difficult to master if a Harpy has not accepted their Monster form (if they are still fighting against their Monster instincts and changes), and is easier to accomplish during the full moons.
Possible psychological changes that come with their respective animals or Harpy/siren lore. How they build their nests, if they might be more inclined to swoop over people, etc. This includes a shorter temper - many Harpies are territorial and aggressive, and can become violent if provoked. The trigger for this temper varies from individual to individual, but it makes them no less dangerous if someone crosses the wrong line.
Changes rundown
MANDATORY CHANGES
OPTIONAL CHANGES