The bird is for a normal x-ray. Here's the context.

CT scans take much longer, but an x-ray is just a few seconds. I think it's just a practical way to get the bird in place for a quick x-ray, and by practical I mean the vet techs minimize their (very real) risk of being murdered.

Birds have very delicate air sacs. For small birds like that constricting them can seriously damage them and cause issues breathing.

Gotta get the wings spread.