Finch killed mate

Question
I had 2 zebra finches. I don't usually cover their cage but did last night because it was cold.When I uncovered them this morning, one was dead. This is the second time this has happened, the 1st was about a year ago. The exact same thing. At this point, do I deduce that it is from them being covered? What is going on?

Answer
It depends on the temperature of the cage. Cages should have three sides covered to help reduce noise and stress. I highly doubt that covering the cage killed him. Is the cage near a window or a vent? Drafts often make birds sick and by the time we see the bird is sick, it's too late.
I suggest buying a thermometer for the cage and keeping an eye on it. My own birds start to look sick if the temperature drops below 26C