Baby chick injury
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You will likely need to find a way to separate them until this heals up or the other one is likely to peck at it. Maybe just use a piece of wire to partition off part of the brooder for her so she can still be near the other one? I would wash the wound well with warm water, use a large syringe if needed, to help clear away any matted blood so you can also see it better to treat it. Plain Neosporin is fine. She should heal up pretty quickly. Not sure what to do about her scratching at it though, kind of in a hard spot to bandage on a bird this size but maybe someone else will have a good suggestion for that.
How big is the brooder you have them in? Normal pecking order scuffles don't generally result in blood shed. Wounds like this are more often from aggression or stress related pecking. Sometimes you just get a mean bird, other times if they are feeling crowded they will start pecking each other.
It's freakinng horrible.