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.
Do not go with a wood base! You will regret it. A wood base will hold germs and bacteria, and will rot away after so many washes and bird droppings. Always try go with a solid concrete base for birds if possible, or a good solid slab base.
You must also be careful of critters digging under and getting through. Ideally you want concrete base, nothing else. I'm not sure why your base is elevated, as you don't give those details, but you need to change that design aspect, in my opinion. Just make sure to even the ground level out under the base.
As a side note, what type of birds are you keeping in that size aviary?