Bowlegs in Chicken

A deficiency of either calcium or phosphorus in the diet of young growing birds results in abnormal bone development. The most obvious sign is bent legs. This can occur even when the diet contains adequate vitamin D3.

Rickets (bowlegs) is seen mainly in growing birds, whereas calcium deficiency in laying hens results in reduced shell quality and lameness. The disorder is commonly known as “cage layer fatigue” and occurs when calcium is mobilized from bones to create eggs.
Another culprit of bowlegs is vitamin D3 deficiency which causes weak rubbery bones and the rib cage that is flattened and beaded at the attachment of the vertebrae.
A determination of whether rickets is due to deficiencies of calcium, phosphorus, or vitamin D3, or to an excess of calcium (which induces a phosphorus deficiency) may require analysis of blood phosphorus levels and investigation of parathyroid activity.

Also, see what causes Bowlegs in chicken