I still don't know why my ferals stopped eating--or, rather, slowed way down--but I suspect it might have been the old plastic food shelter I put over their food bowls. I've been using this one a long time, and any cat owner knows how plastic absorbs smells. Well, this one has had to absorb the extra smells of Leo the Pee-Monster spraying it now & then to assert his "ownership" of the property (otherwise known as my backyard). How would you like to eat where someone else has peed? I know I wouldn't!
I checked the food many times, to be sure--there were no bugs in it (the ants have gone to ground now that the weather's turned cold, and I keep out slugs with a perimeter of salt around the food shelter), no weird smells, no effluvia from sick kitties (a problem I had before when poor Mr. Black was deteriorating on his way to the Rainbow Bridge). And my research about Purina confirms that there hasn't been any contamination of their dry food (they had to withdraw only one type of moist dog food, I think).
So the problem must be the old plastic food shelter. I bought another big storage bin this weekend and will get it fixed up as a new food shelter soon. It's been pretty windy the last couple of days, which blows crud into the food and water.
Oh--the cats are eating again, although the food still doesn't dwindle as fast as it used to. Maybe some of my ferals found someone else to mooch off. :D