In a town of blind people who can sing, ten-year-old Lark is a mute girl who can see. Wow. Talk about hooklines. This would instantly catch my attention--so you've got a lot of goodwill going into this. Let's see how the rest of this holds up.
To survive in a society where music is everything, Lark and her sister make a living by catching and selling songbirds. I think the logic of this is a bit off, unless you're implying that EVERYONE in that society sells songbirds for a living. There must be other jobs in that society, right? Is it that they have to sell songbirds because everyone else makes a living by singing, which Lark can't do, being a mute? (This is a bit nitpicky, but I think your query is at the stage where you only need nitpicking) The birds are coveted not only for their songs, but for their wishes. The phrasing here doesn't work for me. How about "for the wishes they grant"? Is that what you mean? Every kind of bird has a certain wish inside it. This is still a bit unclear--it almost sounds like the birds are the ones making the wishes. This is an awesome premise though! Robins are for health, magpies for trouble For wishing trouble upon others? This is a bit confusing because all of your other "wishes" (health, luck, and sense) are things you'd wish upon yourself. But I don't think someone would wish trouble upon themselves. Is there some better way you could phrase this? , blackbirds for luck, and wagtails for sense Wisdom?. Only one bird can grant any wish under the sun; a nightingale. Very cool.
So when Lark hears a nightingale’s song in the forest, she risks hunting at night with wolves, and even trespassing in another bird hunter’s territory to catch it. This makes it seem like those are the only two obstacles. Wouldn't it work better if you said something like: she'll stop at nothing to catch it, including... (I'm not saying you want to use that particularly cliched wording, but I think it'd flow better if you summarized her motivation and THEN gave examples. If she does she’ll be able to wish for a voice for herself Awesome!, and selling the bird will bring in enough money to keep her and her sister fed through the winter. Flip these if you can. Getting a voice for herself is the bigger reward--it has more of a punch than staying fed through the winter.
But everyone in the town has their own reasons for wanting the nightingale, and their own plans how to get it. You could make this more succinct. "They aren't the only ones out to catch the nightingale." ? And when even Lark’s friends Confusing--initial assumption is that they're helping her, when in reality she's competing with them, right? join the fight Is this the right word? for the bird, Lark will have to decide just how far she is willing to go to get a voice. This is a bit vague. How far she is willing to go in what sense?
WISH UPON A BIRD I love the title, by the way! is a middle-grade fantasy complete at XXXXX words.
Heck, based on your title and opening line alone I'd want to give the first chapter a look. I don't think I've made any comments at all about major structural elements of the pitch--my comments have more to do with ways of phrasing certain things. I think this is an awesome query. I'm writing down the title so I can buy this book if (when) it gets published!