Actually, in this case I'm not sure that the harpies would be a real threat. They have the advantage of flight, but there's also several disadvantages;
- They are very fragile, since hollow bones, little mass,
- Large wings are a large targets,
- They are very light,
- They are incapable of flying with much weight, so any armour is out,
- Claws are a extremely poor weapon against armored soldiers,
- They have no hands or fingers. They cannot easily craft or hold weapons.
All things considered, I imagine even a farmer with a pitchfork would be more than a match for a harpy. Humans have the advantage of mass, strength and weaponry.
A single harpy would be prey to even a common hunter. Sooner or later, the harpies need to rest, and humans are really good at tracking creatures to their nests and waiting for them.
Harpies would only become threats when they gather in large swarms. Imagine a cloud of harpies ravaging the countryside, but even then they would fall to an organised force of armoured bowmen.
Overall, I'm not too worried about harpies dropping stones. There's a limit to how much weight they could carry, they won't be able to do it quickly, and dropping rocks is clumsy and avoidable. Any half-decent archer could just shoot them down.
The harpies would be well-suited for guerrilla tactics, but quickly every farmers field would start hanging up harpy corpses to dissuade attacks. Harpies would probably be considered scavengers or nuisances rather than serious threats.
Methods to fight against them;
- Place a standing bounty on every harpy claw. Make sure that every
citizen is armed with a bow and arrow to loose whenever a harpy
- Employ teams of professional hunters to hound harpies in the
wilderness, constantly thinning their numbers,
- Be constantly ready to deploy squads of mounted archers whenever
harpies populations get too heavy.
- Find out whatever the harpies like to eat, and then hunt that creature to oblivion.
- Poisoned traps seem like a good idea.
- In fact, is there a disease that affects harpies and not humans?
There likely is, somewhere. If so, use it. Perhaps find a bunch of
sickly harpies and catapult their diseased flesh over harpy roosts.
Methods to defend against harpies;
- Have farmers keep their livestock in barns instead of open fields.
This is perhaps the most important. Harpies will prey on your sheep
and cows, so offer coin to allow farmers to build large,
protected barns instead. Maybe they'll invent battery farming by
- Place poisoned spikes onto everything. Literally everything.
- If the harpies like to roost onto tall places, then how about glue
traps to keep them there? I'm not sure any strong enough adhesive
existed in medieval times, but they could maybe develop some.
- Restrict the places harpies can hide. Start chopping down all of the
trees outside of towns and cities.
- Barbed wire aerial walls?
- Place a curfew that forbids citizens from leaving populated areas
alone. They must travel in groups of three to defend against harpies.
- Like any physically disadvantaged predator, the harpies would be
prone towards targeting babies and children. Make sure that parents
constantly supervise their children - maybe even introduce creche
centres to protect the children while their parents are at work.
- At some point, harpies presumably need to mate and lay eggs. During
that season, the humans declare a mass festival - Egg Smash Month.
The humans would struggle to win against the harpies, but they sure could drive them out of human lands. The harpies would likely flock to some mountain peaks to get away.
However, this depends on an important question - do the harpies have any aerial weaponry? Can they hold a shooting weapon that can be fired from the air? I doubt that they would be able to reload a bow and arrow with their legs, but perhaps a crossbow that they load on the ground and then shoot in the sky?
That could be dangerous. A hail of arrows from two hundred feet would do some damage.
Mind, I'd still favour humans to win. If the harpies do have weaponry, they need to have infrastructure to maintain it. If the harpies have an army, they need a mass roosting point to assemble and organise their forces, they need some sort of buildings on the ground to support themselves. That would be the first place the humans would burn.
The problems would arise if the harpies start becoming creative too. What happens if the harpies start dropping diseased carcasses into human towns, or poisoning human fields?
If the harpies are intelligent enough, the very best way to win a war against them would be to make peace. Come to some armistice agreement, assign separate territories and try to make a long-lasting peace. Hopefully it could be mutually beneficial - the humans could craft tools and shelter for the harpies, the harpies could act as scouts and hunters. In the long run, a war between both races is just going to get nasty.