If these fliers are anywhere near human sized, they will not be jinking around in flight. If they have 20 foot wingspans and fly at a slow glide, which is the only realistic way to look at this short of magic or lethal doses of Handwavium, then they won't be all that hard to hit.
I don't think they'd be all that fearsome in massed battles, until they get scary stuff to drop. Like explosives.
They'd be very very effective in raiding supply lines or doing sabotage and commando stuff deep inside your territory. Think of how much of your food supply is vulnerable in fields and barns. How vulnerable your important officials might be to an airborne commando kidnapping or assassination strike. How easy it would be to torch your cavalry stables or stampede your herds by dropping bad things on them. You can't put roofs and volley guns on everything.
Just the cost to the groundlings of having to spend resources on all that air defense will take a lot away from what they can put toward other stuff, like putting together expeditions to attack the birdmen homeland.
In short, I think they'd be better used for strategic bombing and commando raids. More like heavy bombers and paratroops than divebombers or CAS.