Thanks for having your location in your signature. That eliminates some things it could be since you are north of the equator. There are different reasons chickens molt. The best known one is the days getting shorter in the fall but that is not the only one. Juveniles go through a couple of molts as they grow. They just outgrow their feathers and have to grow new ones. With yours being as old as they are, it is not this.
Stress can cause a molt, though this is usually a mini-molt. Not all the feathers fall out and have to be replaced, just some of them. Usually not all hens react to stress and go through a mini-molt together, just some and to different degrees. Stress could be caused by running out of water for a period, a predator, moving them to a new location or major housing changes, adding or subtracting chickens which upsets the pecking order, a lot of different things. Did any of the molts coincide with when you got the chickens and merged the flocks?
Flocks have a certain cycle of laying. They normally lay really well shortly after they start laying and quickly build to a maximum egg production. But after they have laid for a long period production gradually drops off. I’m not talking about one individual hen, they are all different. I’m talking about a flock big enough that the averages mean something. With commercial flocks the average production normally drops to about 60% of peak production after about 13 months of continuous laying. Profit level drops and commercial operations are forced into a decision, force a molt and feed them through a molt or replace the flock.
Ours are not commercial chickens but they still follow this general pattern. After a while their bodies just get worn out from continual laying and they need a break. A lot of times this continuous laying can cause a mini-molt where they stop laying and refresh their bodies. Maybe this had something to do with what you saw if they laid throughout the winter?
A change in lighting can cause a full molt. If the chickens that molted in April were on a supplemental lighting regime where the artificial daylight was longer than the actual daylight and were suddenly taken off of the longer day, that could trigger a full-blown molt. The same thing could happen to the June molters (depending on when you got them) but June has the longest days of the year. That just doesn’t sound right.
Did they have parasites (mites, lice or worms)? Parasites can stress them pretty well and cause feather loss.
I don’t know what caused those molts, there are so many different things it could have been. Even if I were there watching them myself I might not be able to figure it out. Since they are all at least a year old if you do not add supplemental lights they should all go through a full-blown molt this fall when the days get shorter. Different chickens molt at different rates. Some are over it in a little more than month, some can take five months. Generally production breeds molt faster than decorative breeds but again that is general. Each individual chicken has its own molting rate. It’s not about how fast the feathers grow back but about how fast the feathers fall out. That’s controlled by the individual chicken’s genetics.
If you are going to try to stop the molt and keep them laying through the winter you should be getting started with that pretty soon. The days are already getting shorter and could trigger a molt at any time, though most will wait a bit longer to start. What you want to do is to match the length of daylight now so the days don’t get any shorter.
Some hens, especially production breeds, will start laying again as soon as the molt is over, even if it is the dead of winter and the days are very short. But a lot of hens will wait until the days start getting longer in spring before they start up again. Warm or cold weather can have a bit of an effect too but it’s mostly the length of light. Adding lights after the molt is pretty much over can kick start these into laying earlier than they normally would. Just increase the length of daylight by 5 to 10 minutes every couple of days until you are an hour ahead of the sun, then you can stop adding light. Don’t stop the supplemental light until the natural daylight matches your artificial daylight or you might trigger another molt.
Good luck!
