Thanks to all of you who have asked about Lady Bird's condition after her horrible ordeal. She spent the first day dozing and looking miserable, but by the next day she was eating, drinking, pooping, etc. Her wound still looks terrible, but it is dry and clean. We aren't sure what's going to eventually grow over this gaping hole in the back of her head—will she get back skin and feathers? We don't know. But at the moment we're just monitoring it for infection and keeping her away from the other chickens. She is more or less herself–and thank goodness she can't look in a mirror and see how awful she looks!
Our other chick, Ida, is now spending the days outside in the run, inside a wire enclosure that I built to keep her safe from our murderous adult hens. I used a combination of chicken wire and a dog pen I bought at a pet store, with a tarp over the top to shelter her from rain. She comes in at night because it's still pretty cold outside, but the idea is to let her get exposed to the chickens gradually through the bars of this enclosure. We have to keep her away from Lady Bird so she doesn't pick at the wound.
Of course, I only wish I'd done this in the first place–it cost maybe $75 in materials but I would have gladly paid that to avoid this terrible attack Lady Bird sustained.