Zinc is commonly used to apply a protective weather-proof covering for steel and iron intended to be used outside (a process called galvanising).
As chickens are extremely curious and naturally attracted to shiny objects any nail or screw dropped on the ground, perhaps after hen coop repairs, will be fair game and may be swallowed. Tiny snippings of chicken wire discarded during repairs are a similar hazard, so ensure you tidy up thoroughly after any refurbishment work.
Similarly tiny pieces of zinc that are swallowed can cause zinc toxicity and can make hens extremely poorly; as many hen feeders and drinkers are made of galvanised metal they should also be checked routinely to spot any corrosion or damage.
Finally, but vitally, all hen keepers must be aware too that Apple Cider Vinegar (ACV), a natural hen tonic recommended for their well-being, will react with zinc in metal feeders. As such ACV should always be used in ceramic or plastic drinkers to prevent the metal corroding and zinc leaching out into the drinking water.