I'm not totally sure, as I'm not big into treats. I did used to provide egg to a sick bird, but it would be a rare treat for the whole flock. Years ago, I did give them live meal worms on occasion, and went through one bag of a dried mealworm/shrimp (?) blend. Anyway, I am pretty confident it is because I threw lots of pasture seed... several 5-pound bags of chicken forage blend, plus a few 7-pound bags of grass seed, plus additional alfalfa, clover, millet and lentils... all for growing pasture for the chickens. But they ate a bunch of the seed. This was over the course of several months.
Will your girl eat baby bird formula? Egg, sunflower seeds, etc. have lots of calories. Animal protein can help build muscle, I believe. Sardines or tunafish without added salt might be good.