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A poor diet especially one low in minerals like mag and potassium or vitamins like D3 will result in what you're seeing. Again the guaranteed analyst tag off the chicken feed bag may help.
Is that egg shell less or just thin shelled?
Edited by chickengeorgeto - 10/1/15 at 10:18am
Thin egg shells can be a common occurrence, especially in new layers. Make sure that you are feeding layer feed, since it contains more calcium, and put out some crushed oyster shell for free feeding. Many chickens can still have this problem which later can be a factor causing egg yolk peritonitis and infection if eggs break inside. I really like to feed back all of my egg shells crushed up, since the chickens will grab those more eagerly than oyster shell. Today you could supplement her with a calcium tablet or Tums given orally. Both dissolve in a small amount of water also.