Canonicity: "To Be Decided" but likely to be canon, since this lines up with most stories.

In short, Harpies are lazy, crass, opportunistic, flying creatures which look somewhat like humans. Do not trust them to do anything remotely inspired or motivated, and lock your fridge. 

Biology:
A Harpies appearance is a mixture of avian and humanoid biology. Harpies seem to have the torso and head of a human, with the arms and legs of a bird. Much like large birds of prey, Harpies arms are covered in a base layer of soft down, with long feather meant for achieving, maintaining, and stabilizing flight. 
Harpies lower legs are birdlike, resembling those of a raptor or other large bird of prey. The bird-like resemblance terminates at the knee, and instead a human-like thigh connects to the torso. Harpies have a tendency to grow soft, downy feathers over their body in places where a human would normally grow hair, save for the top of the head. 

Harpies height is in a typical human range, typically being within 5”6 to 6”1 in height (however, just like all races, this will vary). Harpies weight can fluctuate greatly, with a Harpies appetite allowing it to put on many pounds of weight in a small period of time. However, a Harpy's quick metabolism will often result in it being able to lose weight quickly, allowing it to still be able to fly despite its overindulgent behaviors.
Harpies have surprisingly strong jaws, having around twice the bite strength of a human jaw. While this does not sound like much, it is actually extraordinarily strong. Harpies are able to crack walnuts, bones, coconut shells, and other solid objects. This power goes well with their teeth, which are sharp, jagged, and very large. Harpies are well known to be able to eat walnuts without even shelling them, cracking open the shell as if it were no stronger than the skin of an apple. 
If Harpies were to have a high protein diet, drink calcium and vitamin D fortified milk, and were motivated enough to bulk and work out, they would be truly terrifying. Their rapid metabolism would allow for microtears accumulated during workouts to be healed nearly instantly, and their ability to eat near constantly would allow for them to pack on weight rapidly. They would be utterly terrifying, and we must be sure that they never are motivated unless we want to be utterly annihilated. 

Contrary to popular myth and belief, male Harpies do exist. Much like elves, male Harpies don't appear as “masculine” as the males of some species, although Harpies can tell the difference easily.  Much like penguins, Male Harpies tend to stick back towards the nest and watch over eggs or slaves while the females go hunt. However, this might not always be the case, as some harpy families have the male go and hunt while the females stay back. 

Harpy reproduction is a mixture of both human and avian practices. Harpies reproductive organs are identical to that of a humans, not having a cloaca like many bird species do. Much like most females of many species, a female harpy will “menstruate”. This menstruation consists of a single, hard shelled egg, laid typically once every 1-3 months, although cycles vary per individual.
Much like all birds, a Harpies egg must be fertilized in order for it to bear children. Once fertilized, a harpy will become “pregnant” and the egg will gestate and grow for 3-5 months inside the mother's womb. During this time, the fertilized egg will grow to a larger size than an unfertilized one. Typically, an unfertilized egg is the size of a softball, while fertilized eggs might be twice as big. Once the 3-5 month gestation period has passed, and the egg and embryo are properly formed, the harpy will “give birth” to the egg, and imprint on it. The harpy will then care for the egg like a bird, keeping it the proper temperature while preparing its nest for the soon to be hatched child. After an additional 3-6 months of incubating, the child will “hatch” and force its way out of the egg (typically with its parents help), resulting in the birth of the newborn harpy. 

It is said in legends that Harpies were originally created when a powerful demigod attempted to create his own army of obedient angels by breeding Man and bird together. However, this attempt was met with ironic failure as he created a bird-like being with a distaste for following orders that won't directly benefit it. 

Habitats: 
Harpies are typically found in mountainous regions with lots of open space. Places such as mountain ranges, deserts, tundras, and rocky coasts are a favorite for Harpies, with the large open spaces and high perches being favored for hunting and nesting.

Behaviors:
Most Harpies tend to have a very opportunistic behavior, a trait that often leads them to mercenary work. Harpies will eat as long as food is available, hunt while there is prey to hunt, and sleep as long as they wish to. They are a creature of their desires, and are often trying to find the simplest, easiest way to fulfill them.  Although not particularly unmotivated, a harpy won’t work unless it feels that it has to. Despite this, given the right price, a harpy makes a staunch, loyal companion, given that the meals and payments coming in are of good quality. Harpies will rarely make friends outside of their species, and often enough any “friendships” made are more for the Harpies gain than anything else.
Individuals Harpies tend to be isolationists, making secluded nests up in mountain areas where they can see potential prey and where intruders cannot easily access. If/when Harpies group together, it usually is only to form raiding parties and hunting bands.

When hunting, Harpies typically choose isolated targets that can easily be overpowered. Although they are raiders, Harpies are not ruthless. Unlike Orks and other species, Harpies don’t particularly enjoy killing, and if they can find an easier alternative they gladly will. Harpies prefer to take victims alive, imprisoning them in their nests where they can either be used as servants or sold for ransom, sometimes in a varying order depending on their value and usefulness.

Weaponry preference:
Harpies preference light, fast firing weapons and explosives for use and raiding. SMGs and LMGs are preferred for pinning down enemies, while hand grenades and Molotov cocktails are often used dropped like bombs from an aircraft.