I, too, have been having problems with egg shell quality in some of my hens this winter. I'm currently trying a change of feed. The problem started several weeks ago and seemed to coincide with a new bag of the same layer ration I've been feeding all year without any problems. I contacted the manufacturer, but I was disappointed in the rather dismissive reply, so I switched to a different brand.
(I'm a veterinarian with an interest in nutrition, so I'm well aware that mishaps sometimes occur in the mixing of animal feeds, but I do expect feed manufacturers to take customer concerns seriously.)
It is also possible that it's a seasonal problem or a combination of management factors. My chicken coop is designed for the hot summers we get here in NC, and although I've winterized it the best I can, it still gets pretty cold in there at night this time of year, so I have a heat lamp on in the roost area. The girls seem to sleep OK despite having the light on all night (they were raised from pups under a heat lamp, and they had the heat lamp last winter as well), but maybe I'm inadvertently messing with their seasonal rhythms in my zeal to make sure they don't get too cold at night. Although egg production has dropped both winters, they have continued to lay since they started laying 18 months ago (at 4 months of age); they're coming up on 2 years of age and they haven't yet begun to molt.
Maybe the thin shells are telling me that at least some of the hens just need to take a break from egg laying. Fine by me; I just want to be sure I'm not mismanaging them nutritionally. We're in a calcium-deficient area, so I've always fed a good quality layer ration plus oyster shell grit (all free-choice). They get plenty of grit for their digestive processes from foraging around the farm, and I haven't seen them show much interest in the shell grit, so I doubt it's contributing much to their calcium needs, which is why I'm suspecting the layer ration - rightly or wrongly.
I'll report back in a few days, when I know whether or not the feed change has made a difference. If it is simply a feed problem, then shell quality should improve pretty quickly.
-Chris King-
