How is zakat paid on a poultry farm that produces eggs? Is zakat due on the eggs themselves, or on their price after sale? Is a full year (hawl) a condition, or is zakat paid every time a sale is made?

There is no zakat on poultry and eggs unless some of them are sold, in which case zakat becomes obligatory on their price if it reaches the nisab and a hawl (one lunar year) passes over it. The obligatory zakat amount is a quarter of a tenth (2.5%).