okay., we had this birdhouse attached to one of our trees near our mailbox.. i knew there were 5 little baby bulebirds in that nest and while going to the mailbox., i seen 3 little bodies laying on the ground., all were dead., and then i seen another one a little ways away., and it moved.., its the only one living., and i cant find the 5th one.., i got it before my cat could., have it in a bowl with lots of rags in it., im trying to keep it warm..,
What should i do with it? I cant just sit it back out there so my cat can kill it.., and i dont even understand how they got out of the birdhouse.., the nest is far down in the house so my cat couldn't possibly reached them., plus hes really fat and can barely climb a tree more than a foot off the ground. What should i do?!?! Please give me answers!

p.s. It is almost bald with tiny hairs(fuzz) just on its head and a few right down the center of its back.