Just give them time to acclimate and recover.
Stop putting oyster shell in feed, keep it in a separate container that is always available if they need it.
They need more protein, 16% is minimum for laying and molters need more.
I'll bet they may be molting....only time will tell.
Hard to say if the bird feed is 'bad',
but it may be diluting the nutrients in a balanced chicken ration,
and you're trying to get some poorly chickens back to optimal health so I'd cut out anything else.
In a situation like yours, I suggest to keep things simple...good balanced ration and lots of fresh water.
My Feeding Notes: I like to feed a 'flock raiser' 20% protein crumble to all ages and genders, as non-layers(chicks, males and molting birds) do not need the extra calcium that is in layer feed and chicks and molters can use the extra protein. Makes life much simpler to store and distribute one type of chow that everyone can eat.
The higher protein crumble also offsets the 8% protein scratch grains and other kitchen/garden scraps I like to offer. I adjust the amounts of other feeds to get the protein levels desired with varying situations.
Calcium should be available at all times for the layers, I use oyster shell mixed with rinsed, dried, crushed chicken egg shells in a separate container.
Animal protein (mealworms, a little cheese - beware the salt content, meat scraps) is provided during molting and if I see any feather eating.
Edited by aart - 11/30/15 at 7:06am