[Finalgenesis]

Animal tag is supposed to confer a penalty per one of the designer, but in reality they make no difference and no penalty or modifier of any kind is applied to the animal tag. someone did some testing sometime back and confirmed this I believe. I think it's the "lies my rulebook told me" thread if you want to read up.

Higher strength units make better siegers, flying also confer a large bonus to sieging, so call of the wind can often make a decent spell for adding siege. The same calculation is true for defenders of the siege, except mindless units get a huge penalty of 1/10 of their original value used. Units with siege or defender bonus add their bonus to the calculation.

You can refer to page 81 in the manual for the formulas in detail, but I think you already know all that and just asking about animal tag... oops.