Why did all 5 of my zebra finches die overnight?

woke up this morning to find ALL 5 of my Zebra finches had died overnight and I have no idea why. They are kept in a cage outside (7ft x 5ft) with plenty of food and water. We have had a problem with mice and/or rats recently so this is the only thing I can think of? We have also had a some cold weather/storms (for Perth) but nothing they haven't experienced before. No smoke, etc as they are outside in the back garden, can anybody help please!?