There are 24 non-kosher birds, and there’s a long chain of transmission to that effect, but if you count the birds in the Torah, the list in Leviticus has 20 birds and the comparable list in Deuteronomy has 21 birds. So where is the count of 24 to be found? The Gemara explains. By including those birds that come along with the other ones (a few times), the count works. Also, a hundred varieties of one particular kind of bird — from within the 24. Plus, in the lists of kosher animals vs. non-kosher animals, the Torah includes the shorter one, whether kosher or non-kosher. Also, the Gemara acknowledges that one can have halakhic expertise or animal expertise or both, but they are not the same bodies of knowledge, and the one studying Jewish law must draw on other arenas of knowledge.