12'L x 5'D x8'H size aviary needs what thickness plywood to hold human weight. Aviary floor is slightly elevated from the ground. Also, please specify how many floor joists it needs to hold the weight perfectly.
A standard residential flooring panel is combination subfloor-underlayment with a 16 inch span rating and with tongue and groove edges. This is usually a 5/8 or 19/32 inch thickness. Thus the joists are spaced on 16 inch centers. The joist size cannot be determined from the information provided.
That is just a starting point. Bird droppings are very corrosive and can quickly degrade such floor panels. You should not only consider a thicker panel, but some sort of topping much more resistant to bird droppings.