You're probably not going to like this answer, but I used to run a wildlife clinic and we used to see a lot of injured and orphaned starlings.
If you are seeing air coming up under the skin, the bird likely ruptured an air sac or two when she fell from the tree or was picked up by an animal. There is no real treatment for it; some survive if the sac spontaneously seals. Many die of suffocation. A healthy young starling should be gaping for food (opening the beak and "begging"). If not, it is clearly in distress.
It will not likely survive the night. Honestly the best thing you can do for it is to get it to a veterinary ER so they can humanely euthanize it.
Hi many thanks for your honest answer. I had read online about piercing the skin with a needle to release the air but as you haven't mentioned that am guessing this won't help?
You can try it, but it will only work if the sac has spontaneously sealed. Otherwise, you have basically just opened up its airway to the outside world. :(
As a side note, starlings are considered invasive in most of the country. Many facilities will euthanize them as soon as they come in.