My finch just died in my hand any 1 no why?

i bought 2 mannikins about three years ago. because i got into birds i wanted to breed them so i got a bird shed with a flight and cages. i have around 30 diffrent birds now. so i got up this morning to go and look at my birds an noteced that it was sat on the bottom of the cage fluffed up. so i held it over a heater and got it worm. then i fetched the bird into my house and put the bird into a cage it just keeps tipping over and flapping so i picked it up and it just died in my hand. i know it was cold last night so put a little oid filled heater on just to stop it from freezing i eas thinking if it was cold that killed it were can i get one of them things were when it gets to a surtain temp it puts the heater on i dont know a name for them but i know people use them plz help thanks