There's only one problem with that theory, this is the wrong time of the year for birds to moult. I'm not saying it can't happen, but I find it highly unlikely that nearly 70 birds would go into moult during the wrong time of the year and not drop a single feather.
This is the slow time of the year for laying, typically. The days are just getting longer and the girls will soon prepare to put in longer hours at the egg factory.
Another thought, their breeds could have alot to do with it as well. I've noticed my production breeds (Leghorns, RIR, ECT) tend to fizzle out faster than my dual purpose and ornamental breeds. There's a reason why commercial egg farms restock when the layers are around a year old, and that's because production usually drops dramatically around then.
One more suggestion if nothing else seems to fit, mice can do a real number on your eggs. Sometimes they'll leave evidence behind in the form of shells, sometimes they don't. If you've got a bad infestation, they get very bold, and will come in during the day and steal eggs as soon as they're laid, or they may just take them at night (ask me how I know this, eww). And mice aren't the only egg thieves, rats, blujays, crows, skunks, possums and a whole host of others love to eat eggs and when there's a steady supply the have no problems dropping by the buffet for a bite to eat.
I hope you get it figured out, that's a big flock to be feeding with no pay back! Good luck!
