Global Alert – Advice On Baby Birds?

It’s not often I have no idea how to handle a problem, but here’s a general request for advice.

Tigi Cat captured a tiny bird, but I succeeded in rescuing it.

It’s too young to fly but managed to get from the top of my bookcase down onto the floor!

Probably by falling, but it seems none the worse for a two-yard drop!

Now it’s safe in a cardboard box in my bedroom.

What am I to do next?

NB – Tigi and his twin and his uncle are NOT indoor cats.

They prowl freely, esp on the roof, whence I’m pretty sure the nestling was kidnapped.