Rebbe,
Today's Gemara speaks about a bird korban being posul if it's a nirva, tumtum or androgenes. The bird that is eligible for a korban is a ben yona or tor, a dove or pigeon. These birds are somewhat small and although I'm not a biologist I don't think they, (or most birds), have external zachrus or nakvus, primary reproductive organs that can be seen. It's difficult to tell a male from a female. How can a ben yonah or tor be identified as a tumtum or androgenes, a creature of mixed gender or indeterminate gender?
B'kavod,
Sam Kosofsky
With regard to determining the gender of birds, I don't have a authoritative answer for you, but my hunch is that just as there are distinctive external signs that indicate the gender of a bird (plumage color, body size, etc.), there are similar external signs that indicate to the well-trained and knowledgable Kohen that the bird is a Tumtum or Androginus.
Kol Tuv,
Yonasan Sigler
This is not a Psak Halachah