Cat has just brought in a bird, should I just put it in a box?

[virgil]

It is alive and just laying still, breathing very heavily. Doesn't appear injured although I have suspicions about one leg.

I think its a wren. I've put it in a ventilated box on some lint from the tumble dryer with some water and some bird seed. Should I just leave it and see whether it makes it through the night?

It may well die over night but by putting it in a box you will have made it feel less stressed. A couple of weeks ago I found a blackbird in a similar condition in the garden. I didn't want a cat to get it and stress it out more so I sat with it until it died - it only took a few minutes but I couldn't bear to let it die alone.