Cows produce milk and usually give birth to one calf at a time. Sheep are also mammals that produce milk, with lambs drinking from their mother; sheep can give birth to up to three lambs. Pigs are mammals that produce milk and can have litters of up to 12 piglets. Chickens are birds whose chicks develop inside eggs and chickens can incubate up to 12 chicks at a time while laying over 300 eggs per year.