We have six ISA brown chickens. Four are two years old and two are new this year. It took some doing to get them together but after several weeks of introduction, they all are in the same coop. The two new girls are laying every day. The older girls stopped laying almost completely for several months but now they're producing one or two eggs a day. The big problem is that one or more of the old girls is laying eggs with very thin shells that frequently break in the nest. Today I had one with part of the shell broken away but the contents still inside the shell. We are feeding them laying pellets. They have a dish of oyster shell and a small amount of oyster shell is mixed in with their feed. When the weather is good, they're out and about for 6-8 hours a day. Now the rains have started so they'll be in their coop more. Is there anything more we can do to toughen up the shells? (The new girls' eggs have very sturdy shells.) My husband thinks they need a winter vacation and we should remove the light from the coop that gives them 14 hrs of daylight. Would that help?