Each winter I have to deal with the same thing with one of my cats.
She has fine fur as it is, but when it starts to shed leaving bare spots, I have to take her in to see my veterinarian.
The problem is usually taken care of, but I still have not figured out why it keeps happening to her each year when the weather turns cold.
Cat hair loss can be a side effect of fleas.
If there is any chance your cats may have fleas the first thing you should do is take care of that problem, then you can worry about the bald spots.
Though some cats just scratch when they have fleas, others have an allergic reaction.
Cat hair loss can be a direct side effect of that allergic reaction.
I have noticed that my veterinarian will give my cat a shot of cortisone when I take her in for cat hair loss.
This leads me to believe she is allergic for something, but the check for fleas always comes up negative. We don’t have that problem.