Well ... you have to defend their attack first, and don't think they wouldn't defend their supplies. It would be very hard to attack the supply storages

@Jannis The supplies are on the ground, so a race that relies on flight for combat can't stand their line to defend them. As for surviving their attack: no attack is undefendable. Even Romans already had infantry formation against attack from above - the turtle. Answer edited to include that.

What about oil + fire attacks from above? No turtle formation can survive that. The supply storages are most likely on terrain that's comfortable to angels, like mountains or plains with no cover. And you will need your soldiers to defend your people. Another thing is that the angels, without a supply storage, will rob your defenseless farmers. You try to beat Mosquitos by destroying a blood storage near you. But instead all Moskitos die they will suck you empty, because you have no effective strategy against them

Angels raiding those 'defenseless farmers' will quickly discover that the moment they land to actually do the raiding, those farmers are anything but defenseless. Anything those angels could carry off would be kept in a structure that the angels will have to enter in order to raid. Inside those structures, their flight advantage is gone, and their muscle mass and technology disadvantage is multiplied as a result. A simple pitchfork would leave an angel with crippling wounds, preventing it from fleeing even if it were to completely give up on taking anything as part of the raid.

@TheoBrinkman These Angels are warriors. That means they're definetly trained to hold a weaponand use sword, spears and more. Untrained farmers won't stand a chance against the whole army. And I think they can carry more than an oil lamp, I rather think of two angels holding a whole bathtub of oil or tar. Even when both armys fight it's expected for the angels to win

@Jannis It's not matter of training, it's matter of biology. Well trained 500g pigeon can carry about 75g payload. That's about 15%. 72kg angel could optimistically carry about 10kg, that budget includes clothing, the tub and the oil. Flying creatures cannot share load the way ground creatures can. "Two angels holding" is not fiction, it's "people instantly recognizing it as impossible". If one handwaves aerodynamics away, there is no point in giving them wings at all. Magic-hovering genies work way better in such scenario because they don't contradict daily knowledge.

@Jannis, a creature with a 4-meter wingspan, inside a barn or other such structure on a raid loses all of the maneuverability advantage granted by those wings. The muscle mass disadvantage imposed by the small size of that creature means that the average peasant is as strong as 3-4 of them combined, and any significant damage to a wing leaves the angel completely unable to retreat. You've missed or forgotten that the bulk of medieval armies were conscripted peasants. Those same farmers who knew how to use a pitchfork as an effective implement, and hunted their own meat with bows.

(cont'd) Your posts presume that the angels are expected to win, and rely on that presumption as their evidence. The angel's standard combat load can't be more than 10kg in total, including clothing (about 1kg), any containers to make carrying their gear easier (about 1kg), weapons: let's say a spear and 4 darts/javelins (about 6kg), that leaves them 2kg of additional carrying capacity if completely unarmored. Unless they leave their weapons behind after a raid, they're not even taking a goat on those raids, because they won't be able to fly away with that much load. Flight != win.

(cont'd) Additionally, in a world where your medieval peasant can reasonably expect to be attacked from the air, those same peasants would learn quickly to live in structures not innately susceptible to those attacks. Molotov cocktails drop from the sky? Build houses in hillsides, with earthen overhangs protecting doorways and windows. Darts dropped indiscriminately from a kilometer up? Include a layer of flagstone under a foot of earth as the roof to those houses. Do the same with barns, grain silos, etc. Suddenly, to be any threat at all, those attackers need to be on the ground.

Why are bird people now so small? And why shouldn't they be so good with the sword, that they can just kill the farmer? Two things when in combat with a bird warrior: They're very agile with the wings on an open field (even when not in air). When defending as a group against bird warriors, you may take the agility, but the group is very vulnerable against splash(area) damage, like molotov cocktails to be stupid. And why should bird warriors be worser with the sword as humans?

@Jannis, Still further, the capability of the weapons available to the non-flying side are not symmetrical with those of the flying side. The angels can't deploy siege engines, or even heavy weapons at all, simply because they can't transport them without being stuck on the ground.

@Jannis, the angels are so small because they have to be in order to fly with a 4 meter wingspan. Wings don't make you magically agile. Especially not on the ground, where they are either folded up in order to be kept out of the way, or open, which makes you a big target with your most fragile (and necessary) body part the biggest and most easily hit target. Farmers don't keep their stuff in "an open field", unless we're talking about crops that have yet to be harvested. Any other target for a raid (remember, you were talking about raids) is going to be stored inside a structure.

(cont'd) Again, that means the angels have to sacrifice every physical advantage their wings give them in order to be able to potentially run off with as much as 10kg of 'stuff' per surviving, uncrippled raider. Molotov cocktails deployed during a raid will, inevitably, end up destroying the target of that raid, whether you're talking crops in an open field, or goods stored inside structures. Also, they're remarkably ineffective against anything but relatively stationary objects or tightly packed groups, and are more dangerous to a creature with large expanses of feathered wings.

Well the machines can be destroyed with fire, pretty easy, since most things in an medieval settings is made out of wood. And @TheoBrinkman in the question stands 'humanoid' and 'slightly lighter than humans'. I wouldn't think of smaller creatures at all. And farmers won't protect the houses that strong, that they build in hills, maybe and only maybe castles are especially protected against bird warriors. And what you completely forget is: Bird warriors are so fast, they will rush and kill the man on their open own field, and kill the mostly unarmed woman and children in the settling

@TheoBrinkman Why did you thinks farmers could resist against bird warriors, while castles full of knights barely can? And, the bird warriors can shoot with bow and arrow pretty easy every not protected farmer down

@Jannis, again you're presuming that the non-flying humans are stupid and that wings make the angels magically strong/fast. The size (mass) of a flying creature with a 4 meter wingspan has an upper limit. That's what provides the disadvantages that you're refusing to even acknowledge. Actual birds exist with wingspans in the 4 meter range. They can't carry enough extra weight to be any significant threat to a human. (Wandering Albatross: 3.7m, Great White Pelican: 3.6m). Why do you think that "castles full of knights barely can" when it isn't supported by any of the other answers?

@TheoBrinkman The bird warriors are magically strong... The asker tagged magic and wrote as answer to my question that this magic is used to make the bird warriors fly.

@Jannis, assuming the angels carry nothing else, they're going to be packing a 7 liters of lamp oil in bottles of some sort. Yes, that would be a great way to instill terror on a small village of unprepared people living in wooden structures. No, that's not going to allow the angels to win anything resembling an actual battle unless they severely outnumber those they are fighting. All of your claims presume that humans are incapable of adapting to what happens to them, and that angels are impossibly fast/strong for the parameters of their physiology.

(cont'd) From the first line of the question: "Bird people are humanoid creatures with a wingspan of 4 meters, they are as tall as humans and the fittest of them weight 72 kilograms or slightly less, so they are quite lighter than the average human."

@TheoBrinkman The Birdwarriors are as strong as normal humans, when flying they're much faster as a human. They are simply humans that can fly

@Jannis, that's directly contradicted by the information in the question (I even quoted it for you). The fittest angel weighs 72 kg (about 160 lbs). That includes wings and flight muscles, which detract from the muscle mass available for any non-flight activity. Those flight muscles have to be strong enough to lift a 72 kg body and fly. The muscle mass available for non-flight activities is significantly less than the same mass for a non-flying creature to do those same activities.

In fact, since the existing birds with wingspans approaching 4m have entire body masses in the 10kg range, it is questionable whether these angels could even get airborne at 72kg, much less be able to wield a weapon of any sort. (This is the real problem with the notion of 'flying humans' in the first place.)

The simple fact is that a 72kg, flying creature has virtually no muscle mass left available for non-flying activities. The estimate of 10kg carrying capacity was unbelievably generous, but still limits their capabilities well below the point where ground-based civilizations would be helpless before their might.

Try watching this video (youtube.com/watch?v=-6h65t2MOfU), for an example with the difficulties of accurately dropping an object over a distance of less than 200m. Then take into account the multiplied inaccuracy of a dropped object from an altitude of 1km or more.