Injured Wild Common Starling

I found an injured starling by the side of the road, and for better or for worse I took it home (and put it in a cardboard box with a t-shirt and a metal grate with a towel covering it). It doesn't seem to be able to move either of its legs; one is curled up and tucked in and the other looks pretty limp, but its wings and head move just fine and it has no other visible injuries. I think it went into shock for a bit because it was breathing pretty heavily with its beak open and its feathers fluffed up, but it's calmed down a lot and seems alert (I set up a bird nanny cam so I could leave it alone). It tried to push itself up with its wings a couple times and ended up flipped on its back, so I put another t-shirt next to it so it will stay put.

I've called about 50 wildlife rehabilitation centers and a handful of vets in my area and have come up empty-handed so far; most of them went straight to voicemail. It's probably been about 5 hours now. I don't think I'm likely to get a response from anyone until tomorrow, since it's nearly 8pm here now, so I need help with what to do until I can get it into some more capable hands. I read that I'm not supposed to feed it or give it water, and I'm not sure it could eat or drink by itself anyways since its movement is so limited/it's probably really stressed, but I'm worried about how long it's going to take to get it to a rehab center.

Worst case scenerio, if I can't find anyone to take it, would humane euthanization be the best option, or is there anything I can do myself to help it survive? Starlings aren't protected under any conservation laws here and are legal to keep as pets; I'd be willing to set up an aviary for it and take care of it if it can't be fully rehabilitated but would have decent quality of life.