Weak shells - the cold or calcium deficiency?

[SoManyHats]

I switched my laying hens to grower feed because I now have a mixed age flock. I put out oyster shells also. The last week or so, it seems the eggs shells are very week- could that be because of the cold, or should I worry that thy aren't getting enough calcium?

 

[ChickenCanoe]

I made the same switch about 6 months ago. Just make sure you are providing more oyster shell in a separate container than you did before. I even have a container next to the nest boxes in addition to one next to the feeders.
I initially had a couple thin shells but now I'm not seeing any difference from before I went to grower.
The cold should have no impact.
When the egg enters the shell gland, the hen pulls calcium from the medullary bone to build shell and that is replaced by dietary calcium.
The egg is most often in the shell gland at night. Large particle oyster shell is superior to layer feed calcium since the large particles eaten in the afternoon are making contact with the calcium absorption sites in the intestine when needed. A chicken can assimilate calcium in the intestine faster than almost any animal.

Phosphorus and vitamin D3 are almost as important as calcium for egg shells since the hen can't utilize the calcium without the other 2 being present.
A good poultry feed and exposure to sunlight should take care of both.