Hullin, Chapter Twelve, Mishnah Two
1) An unclean birdone is not obligated to let it go.
2) If an unclean bird was sitting on the eggs of a clean bird, or a clean bird on the eggs of an unclean bird, one is not obligated to let it go.
3) As to a male partridge:
a) Rabbi Eliezer obligates [one to let it go].
b) But the sages exempt.
Section one: The obligation to let the mother bird go applies only to a clean (kosher) bird, one that can be eaten. It does not apply to birds that are not kosher.
Section two: Furthermore, if either the eggs are from an unclean (not kosher) bird but the bird is clean, or vice versa, there is no obligation to let it go. The obligation applies only to a clean bird sitting on clean eggs.
Section three: The Torah says mother because most of the time the mother bird sits on the egg. However, the male partridge does sit on the eggs, and therefore the rabbis debate whether there is an obligation to let it go. Rabbi Eliezer says that there is. Evidently, Rabbi Eliezer believes that the Torah mentioned mother, because that was the normal case. But according to his opinion, the same rule would apply to the father as well.
The other rabbis are more precise in their reading of the Torah (and more hesitant about extending its logic). One must let only the mother bird gothe father bird may be taken even while sitting on its young.
As an aside, this reminds me of the debate concerning uncle/niece marriage. The Torah forbids a nephew from marrying his aunt, but says nothing about uncle/niece. The Jews who composed the Dead Sea Scrolls forbade both cases, thereby extending the logic of the Torah, much as Rabbi Eliezer does in this case. The rabbis, on the other hand, lauded such marriages, and read the Torah in a more narrow fashion.