Today we see that if you want to monkey with known concepts, you have to get the reader to trust you before you start laying down the weird.
What I gleaned about the story: A young bird/boy/thing fails his fledging test and tumbles from his sky city home to the scary world below. Presumably he will not be immediately eaten.
Find this book on Amazon.
Analysis: So the protagonist is a young talking eagle, complete with wings, feathers, talons, a beak… and hands. That threw me. His fellow sky-dwellers are ravens, owls, albatrosses, etc. Except they’re not, really. Because they have hands, too. Do these hands grow out of the ends of their wings? Do they have entire arms, in addition to the wings? I don’t know. And the fact that I can’t even picture the body shape of the characters is a big distraction for me.
Analysis: So yes, birds do actually have knees, but they kinda don’t. The knee joint on a bird is tucked up under the feathers. You almost never see it. The visible joint in the middle of a bird leg–the joint that actually does the work to support the body’s weight–is the ankle. If a bird is scared and worried about losing his ability to stand, it would be his ankles that got weak, not his knees.
Now, I understand that “weak in the knees” is supposed to convey fear. But that’s a human term. Bird people wouldn’t say that. And when an author uses default idioms of English that wouldn’t really exist in their story world, it pops me out of the story. A realia is a word that exists in one language, but can’t be translated directly into another language. The phenomenon in this story, however, is sort of the opposite of that: words that do exist in the fictional world/language, but shouldn’t. So I’m going to call them anti-realias. (Unless somebody contacts me and tells me there’s a better term already in use.)
Analysis: The story establishes that these bird people are still hatched from eggs, and that the young are flightless. But instead of being fledged within a few months, like the birds we know, these bird-folk fledge at age fifteen, and this popped me out of the story. Really? A species that still fights and competes among themselves in the air, and juveniles can’t fly for fifteen years? This would be like human children being unable to walk until they were old enough to drive cars. Society would be crushed under the burden of lugging all those large, flightless children around.
Ultimately, the problem I had isn’t really whether these bird folks have knees or ankles, or whether fledging should happen at fifteen weeks or fifteen years. The problem is that the author has not given me a clear picture of this species, so I’m having to piece it together from the word choices, etc., but I’m getting mixed signals. I believe that if you are going to alter a fundamental concept for which the reader already has a well-established understanding, you have to hang a lantern on the fact that you’re changing it. You have to acknowledge to the reader that, yes, this is not the <concept> as you understand it. I’m making changes, so bear with me. If you don’t do that, and quickly, the reader is left to wonder whether you’ve ever even seen a <concept> before.
Several years ago, a friend of mine wrote and produced a vampire movie for Canadian television. (Rufus) Except, the protag wasn’t really a vampire. He just kinda quacked like one. So when my friend and I were discussing an early draft of his script, I suggested that since Scene 1 had this Rufus character stepping off a bus in the middle of the day, he needed to lampshade the whole “vampires and sunlight” thing, or he was going to lose the trust of his viewers. To do this, we decided that after watching the boy step off the bus, we would cut to a shot of the sun, blazing brightly in the sky. And that was all that was needed. That one shot said, “Yes, I know you think he’s a vampire, and yes, he’s standing in the bright sunlight. But I’m showing you the sun now so that you know I did this intentionally. It’s not a gaffe by some schmoe who doesn’t know what vampires are supposed to be like. I do know. Now cut me some slack and trust me.” So folks did.
But that’s what this book did not do with the bird creatures.
I think the problem here is mainly a result of the author either aiming for or not thinking beyond a niche audience, combined with you not being a part of that audience. I’m only passingly familiar with the anthropomorphic animals genre, but it was enough to give me a frame of reference so that all of the elements which broke your immersion here were known concepts; instead of throwing me out of the story, they would have only functioned to signal what kind of story to expect.
In short, these are not so much your traditional talking animals ala Tailchaser’s Song or Mrs. Brisby and the Rats of Nimh as they are fully human-shaped beings with relatively humanlike anatomy and developmental stages. The characters can be assumed to have knees, elbows, ankles, etc., in the places where you would expect them on a human character, and they’re probably walking plantigrade unless specifically indicated otherwise. Female characters may or may not have breasts, but everyone is probably wearing humanlike clothing with minor alterations to allow for things like wings and tails. There are probably no actual humans in this world, but if there are multiple species they’ll most likely all be fairly similar in size and build. A hummingbird-person might be shorter, slighter, and more agile, for example, but wouldn’t be dwarfed by an elephant-person to the same degree as their non-anthropomorphic counterparts would.
The hand issue with avian characters is still potentially a question at this point, but none of the potential solutions- hands on the wings, cartoon-logic wings *as* hands, or separate arms and wings, like an angel- would be particularly unexpected. (A fourth solution, giving them no hands but dexterous feet, *would* be a bit of a surprise, but still more an unusual spin on the trope than a source of cognitive dissonance.)
This isn’t to say that your points aren’t valid, but I think the fault is as much or more with the marketing and presentation here as it is with the actual writing. A blurb specifically describing the cast as anthropomorphic, for example, or a cover showing us our protagonist would go a long way towards mitigating this kind of assumption mismatch.