What should i do if i found a little bird fall out from the nest

I found this little bird on our balcony yesterday twice. I put it back to the birdhouse (picture 2) and we put pillows on the balcony, so the little bird doesn't get hurt. My flatmate wrote me in the night that birdy is back to the balcony. My flatmate and me searched after some advice on the internet....what should i do if i found a tiny bird...but we realised too what we should do...so here come some suggestions for a similar case:
* Which kind of bird? - We discerned that this little bird is a tit;
* How big is it? - if it was a really little baby bird, they could not be alive after leaving the birdhouse
* What do they need? Let it there or put it back to the birdhouse? -At both times i put them back to the birdhouse but the third time i realised that they don't need it. Luckily the siblings and the parents of this bird are probably not intolerant against my smell (because i touched the little bird)
* What should i do then? - The parents came back and fed their child with fresh hunted insects, so I should do nothing just observe the little bird.

[picture 2]