Vitamin D is very important for chickens.

Luckily, chickens can absorb plenty of Vitamin D from the sun, but if this isn’t possible due to weather or other reasons, then Vitamin D should be added manually to their diet.

Vitamin D is found cod liver oil, as well and is necessary for helping chickens metabolize calcium and phosphorus. A Vitamin D deficiency can lead to easily-breaking eggshells or laid eggs to have very thing shells overall.