There is a new empire trying to conquer the world and it is populated by an avian species (looks like an early bird-dinosaurs with bigger head) and their first target is a human kingdom
Abilities :
They are omnivores
They are roughly 4 feet tall with claws on each finger (1 inch claw), weigh on average 20-25 kilos
They can hold objects and weapons and have roughly the same intelligence as humans ( ignore the impossible biology )
2 Km 1 way flight, around 60 km/hour hovering speed (max at 100 km/hour) at an altitude of 100 meters above the ground (can't gain high altitude, although you can get around it by flying from a higher place) and a maximum baggage capacity of 5 kg with 50% flight range reductions
They can sprint at 40 km/h for 100 m without weapons on hand (like a quadruped, weapon strapped at their back or waist, so they are easily ambushed while charging), they lose some momentum because they have to draw their weapons upon contact, but not much. Their walking speed while holding weapons are roughly 1 km/h (slower than human)
They tire easily (20 minute battle and that's it) but they also recover their stamina pretty quick (1 hour rest)
THEY ARE WEAK (a shield bash would kill them, for comparison, let's say in an open battle, a trained soldier can kill 4 of them.)
Their senses are a bit superior than humans but treat it as if it's the same
Armies :
The avians are still in their home island archipelagos, with their a bit superior navy while the humans are unprepared (roughly a week before humans can really gather their armies)
They have a very small amount of dragon riders (4 meter long carnivorous lizards (can't bite trough metal), have a travel distance, speed and strength equal to human horses, breathes fire (once a week kind of thing, so no abuse), immune to arrows at over 50 meters, they can fly until 1 km above ground and let's just assume they are completely loyal) *ignore how they are able to tame the dragons
But they don't have any heavy infantry, or ground cavalry. Most of their troops are made of archers
The humans have 5 times the population and army size. They do have deep rivers linking their major cities
Numbers (if you want a solid calculations) Avians have 2.000 men, all are completely trained, total population is 400.000 and only have 20 usable dragons Humans have 10.000 men, 60% are conscripts , total population is 2 mil and it has hills and forest everywhere.
IN SUMMARY: How do you defeat over a kingdom with air superiority but only having archers and light, unarmored light infantry as your ground forces. ( I want the avians to win)
Notes:
Supplies are complicated, so lets say both sides are self sufficient and are able to constantly transport supplies using ships/caravans for a 3 year long war. Avians eat less than humans
Just assume the humans don't have any allies helping
Both of them have classical age technology
Avians cannot 'shoot' while flying on their own (although a couple skilled one can if you want to add a fear factor), they can only drop stuff.
Assume that the avians have the same technology that humans have, or at least an equivalent of it (for example: saddle-Dragon saddle)
They can communicate with each other, so humans surrendering and being a vassal/ enslaved/ giving some land to end the war is possible