The most "famous" tax deductions are social engineering created by Congress to encourage certain behavior, like having children or owning a home. But the real purpose of a tax deduction is to go against income -- if you earn $5000 but you have $1000 of expenses, you should only have to pay tax on $4000 of profit.
So, if you raise chickens as a business and report taxable income from selling eggs or meat, then you can deduct your expenses (food, chicks, transportation to the farmer's market). But if this is a hobby or personal use and you don't report taxable income, you can't deduct expenses. Congress never created a special chicken deduction.
