Sounds to me like they were in the heat, and then got cold? The black spots at frost bit, not mites. If they have worms, the will get weaker and weaker over time, Not in a day or week.
Mites would have to engulf the entire bird for death and you wouldn't have to guess, You would Know. Some of our rescued birds have heavy mites and have probably lived years and years with them. Not likely a flu in the cold. As long as you have 4 birds, a place for them to get off the ground and out of the wind, they will do better without the heat. If you give them any heat you will have to always give them heat. We had heated coops and lost some every year. Now, they get none and rarely do they die. Just pile lots of straw in a corner of their coop and they know what to do from there.
If you think you have Mites, and your feeding them organic feed? why would you use seven? we just use a little skin so soft and if we get a bird with lifted leg scales, we dip their legs in old motor oil a couple of times in a week, it smothers the mites and kills the eggs. Taught that by an old farmer. we tryed vasoline but it don't get up and under.
It's always possible you just had a bird die from a heart attack and the others are fine? At about a year old is when we see that type of thing happen? If the rest are eating alot it could just be they are cold and are just trying to keep their furnace's running?
If they are eating and drinking, they'll be fine, God Bless
