Hi Wendy. My son's just come in from work and said the best thing is to get some Hills A/D. The vets should stock it. I'll warn you it stinks to high heaven!!!
Around 2ml from a syringe every other hour should be fine. If the bird does well and gets bigger, then you can start mixing in some mealworms in a few weeks.
We've had allsorts here that have been brought into the vets and needed 24 hour care. Problem is my son then goes to work and I do the care all day long.
The one thing that may happen obviously is that it could imprint.
You could try the RSPCA but they are pretty crap. Today my son's workplace had a call regarding a dog that had been tied to some railings and left. The RSPCA said they wouldn't come unless it was left for at least 2 hours. In this heat, the poor thing would be dehydrated or worse, so one of the nurses went up to collect it and bring it in to check for a chip.
Anyway, good luck with the little love.