Summary: 

Both harpies and sirens are species mainly composed of females, with males being almost considered a rarity. 

They mostly look human, with a few bird characteristics here and there. Beaks, jet black eyes, long feathers instead of hair, wings for arms, with a few claws at the wrist that they can use like fingers, and bird-like legs, feet, and tails.

Reproduction: 

Due to an imbalance in the male and female population in both species, both will partake in polymerous relationships, sometimes with a single male for almost 10 females. 

Harpies will lay eggs in intervals of at least 2-3 years, laying 1 or 2 eggs at the time, that take around 5 months to hatch. Sirens, on the other hand, will only lay one egg per breeding season, that 4 months to hatch.

Biology:

Chicks of both species are practically identical to the untrained eye, but harpy babies tend to be bigger than their siren counterparts, with siren chicks being slightly chubbier and having shorter necks.

They grow practically at the same pace and both reach maturity at the age of 16, but harpies live up to 80 years, grow taller than most adult humans, while sirens only go up to 60, barely making it past 1,50 meters.

Sirens keep their white feathers, the black splashes on their wings becoming more noticeable as they aquire a plumage similar to a seagull. Their tails grow two particularly long and thin feathers that stand out between the rest, almost like ribbons.

Harpies chicks will gradually lose many of their white plumage as they grow, only the feathers on their legs and lower stomach keeping their color, while the rest turn black and gray with a hint of discolored brows. The feathers on their head grow wide and dense, sometimes blending with the male’s “beard”, making their face seem much wider than it actually is when they puff them up. They also form a couple of particularly large feathers among the ones on their head, making them seem as though they have bunny-like ears.

How to tell the difference:

Besides the striking and obvious differences in plumage and size, they also tend to live in different environments, with harpies settling their communities among the highest trees of rainforests, and sires settling by the coast of warm tropical seas, making their villages on platforms over the sand and shore. Their feet are also different; harpies have large sharp talons on their feet that they can use in combat and hunting, while sirens have webbed feet similar to ducks, that they can use to give themselves a little boost when diving or swimming. 

Additional facts:

Even though both harpies and sirens are more than intelligent enough to master agriculture, the way they build their villages makes it really hard for them to keep cattle or farms, so they remain mostly hunters and gatherers. However, they mastered sustainability, only harvesting enough to live a healthy life with little excess, and avoiding to reproduce more than the minimum required to rejuvenate the population. 

Despite this generations old life style, larger villages began efforts to alter their habitat, controlling the local natural predator populations that compete with them for food, and putting the seeds of all the fruits they consume aside to spread around later. 

Some harpies' wingspan reaches almost 3 meters, with the smallest one recorded so far being 2,45 meters, but even the smallest harpies can use their wings to create gusts of wind capable of sending people flying, with records of one harpy using her wings to unearth a line of train tracks that were built in their territory without their authorization. They even have competitions where they’ll compete to see who can send the biggest boulder flying with their wings.

Sirens use their voices to attract the local wild live with mating calls when they’re hunting; and fish by tying nets to their feet and diving in the middle of schools of fish or using crab traps, but when fishing in a river too shallow to dive safely, they’ll put a net barrier up and wait for fish too big for the holes swim into a fish trap. Much like harpies, they also have competitions to test their abilities, like seeing who can dive the deepest and surf the biggest waves.

The first generations of harpies and sirens were created together, creating a foundation for a rather natural alliance, that persisted even though they live in different environments.

They share the same language, Helinarius, only with a different accent, and hold similar traditions when it comes to marriage, festivals and religious ceremonies. 

Trade between them is also common, but mostly consists of trinkets, paints, and decorations.

Interspecies marriage between harpies and sirens is widely accepted among their populations, with many also being open about unions among sirens/harpies and a creature of totally different species, even if these unions won’t result in fertile offspring or offspring at all.

Personal experience:

I visited both harpy and siren villages, and besides the way they dress, build, and what they eat, I barely noticed any difference in their culture or behavior. They were all pretty welcoming and bombarded me with questions about where I was from and about Earth during my stay. I could barely get any of my own questions in, and I had to prolong my stay because of it. But, on the bright side, I was able to make a couple of very lucrative trades. I got hold of some pelts for Christopher, for when we have to go to colder places, some beautiful pigments for my caravan, and even a couple of really beautiful knives, and still left their villages with my wallet even fatter than when I walked in.