Chickens like all birds lack the ability to digest lactose.
As the other commenter noted, lactose is likely to pass through the chicken's system essentially undigested, and that can cause gas, too. Milk is not the best thing to use to supplement calcium for your chickens. Crushed oyster shell is much better, inexpensive, and readily available for poultry use. Check at your feed store.
I feed my chickens RAW MILK daily.
Raw milk, unlike the pasturized milk that you buy in the store, is safe for most lactose intolerant people (and chickens) to drink.
My wife is lactose intolerant, but she can drink all the raw milk she wants.