Yosef Karo

“ The lion is capable of Drisa even on the largest bull or largest wild animal. The wolf cannot do Drisa on a large cattle, not even the smallest of them such as calves. But it can do Drisa on small beasts, even the largest of them like the largest rams. Cat, marten and fox cannot do Drisa on large rams but they can on kids and lambs, and all the more so on birds. Rat has no Drisa on any beast, even kids and lambs, but it has Drisa on all birds, even the largest. Dog has no Drisa even in birds. ”

“ What's considered a mark of Drisa? When the flesh is reddened facing the intestines, or if the flesh is decomposed facing the intestines, to the extent that's it's like flesh that a doctor would remove. If that flesh looks like it's missing part of it then it's treif, and if it was Dores on the Simanim (esophagus, trachea) , as soon as the simanim themselves turns red at all it's treif, even if the windpipe has any redness.
17 A beast that was Nirdas has no remedy by checking because the Veshet, esophagus, cannot be checked externally. ”

“ And whenever it's definitely treif, even if it's still alive after 12 months it's forbidden (Rashba #98, R' Yerucham and other achronim) . 19 There are some that say that small birds, redness can't be seen in them therefore there's no checking them. Likewise there's those that say the hawk's venom isn't very red so there's no way to check it.
20 If there are many Nirdas livestock it's forbidden to keep them 12 months lest a stumble happen (and the custom is to be strict even with one) .
21 It's forbidden to sell a Drusa or Safek Drusa to an idolater lest he sell it back to a Yisrael. ”