Owing to the efficient bio-addition of eggs with vitamin D by natural sunlight exposure of hens, we hypothesized that commercial free-range eggs produced in summer should contain higher amounts of vitamin D than barn eggs or free-range eggs from winter.
However, the vitamin D analysis did not support our hypothesis. After visiting the farms, we assume that the freerange housing conditions are crucial for the choice behavior of the chickens.
An important aspect that could influence the movement of chickens in and out of the hen house is the offered space for feeding [8].
We observed that chickens in a free-range husbandry avoided moving outside if they received their feed exclusively inside the barn.
Besides the feeding points, habitat preferences of free-range chickens are also important.
One study observed that the number of chickens found to be ranging outside was positively correlated with the amount of tree cover the range area contained [9].
Thus, the efficiency of sunlight exposure to produce vitamin D-enriched eggs also depends on
the habitat conditions.