It wouldn't be much of a backpack for the bird, you would be standing on your knees on the bird's back when firing bow. Remember, elves have bows. Otherwise, you couldn't aim and would be likely to injure the bird when firing. It would be a sort of a sled, a small platform strapped to the bird, with straps to fix your ankles to it. You lean forward when travelling to reduce the drag, and stand up in combat to act as a turret.
this is more or less what i was thinking, you'd be in a kneeling position kinda like when on a motorbike but more compresed, then when firing you'd be sitting up
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< <- firing
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< <- all other times.
like that but you're legs would be at a more acute angle,
the straps would either be non-existant for really srrong elves to allow them to bail out very quickly if need be or for less stron users would be tied with a quick release knot
since elves don't use leather it would have to be wood and ropereed, probably featherwood to reduce the weight.
armor for bird riders would also have to be very light.
as far as control, im sure the bird would fly under it's own violition most of the time (while the rider is firing) giving the rider warning (probably by the rider reading it's movements) as to what it will do, when the rider isn't firing probably reigns, vocal commands could also be useful as a backup, though wouldn't be that reliable in the din of battle, perhaps a whistle that changes pitch with how hard you blow it? different pitches and combos would mean differnet things, just have it on a lanyard around the neck and hold it in your mouth during combat
Also I don't know if the Eagle would slice heads off with he talons. I'd think they would be graspers so to speak in that they attack with their claws by grabbing and/or raking
Yeah. They tend to just plunge their talons into their prey's skull, or spine. I doubt they'd really be much good for fly-by attacks, since they'd just get the talons stuck, and tumble comically to the ground. They'd be pretty good at pouncing on an enemy, and glowering menacingly for a few seconds, before taking off again. Assuming the rider could convince them to stop eating their kill.
Have you ever seen an eagle or hawk attack in real life? I have. If a person was in metal it's be OK, but if not... If it's a large or giant variety of those species (and you ignore the implications to flight) if it doesn't shred you to bits and drive it's beak into your skull they will indeed pick you up and drop you to your death then tear you apart. It's one of the reasons why small dogs are a bad bet where I grew up. (Cats do OK though.)
Well, (pre)historically, eagles that preyed on hominids usually did so by killing them there and then, and tearing off chunks to take home to baby. Obviously it depends on both the size of the eagle and how much you're willing to look the other way with regards to the square/cube rule. Given that we've got spiders bigger than horses, I guess you've got a point.
i rather imagined they'd do it like the Nazgûl-birds or whatever you want to call them (the big wyrvyn thingy the witch king flew around on) in the LOTR, essentially swoop down grab a few enemies and bowl a bunch of others over if they are in a group, prefferably off a wall, than go to a great height and drop the ones it grabbed. if all else fails, beak em.
edit: i'm putting way too much thought into this aren't i