Chullin, Chapter 12, Mishnah 2. This chapter is devoted to the mitzvah of shiluach haken, sending away the mother bird before taking the eggs or the chicks. Our mishnah defines the boundaries of the mitzvah: which birds are included, and which situations fall outside the obligation entirely.
A Non-Kosher Bird Is Exempt
The mishnah opens: "ohf tamei patur mishale'ach," a non-kosher bird is exempt from being sent away. Someone who comes across a non-kosher bird resting on its nest has no obligation to shoo it off before taking what is beneath it.
Where does this limitation come from? The Torah says, "ki yikareh kan tzipor lefanecha," when a bird's nest happens to be before you. The Torah chose the word "tzipor." That term is reserved for kosher birds. When Scripture wants to speak of birds in general, kosher and non-kosher together, it uses the broader word "ohf." Since the verse of shiluach haken says tzipor, the mitzvah is speaking only about a kosher species.
The practical result is that such a bird may simply be taken for one's own use, even while it is sitting over its eggs or its young.
When the Mother and the Young Do Not Match
Two crossed pairings follow. In the first, a bird of a forbidden species has settled over a clutch belonging to a permitted species: "ohf tamei" resting "al beitzei ohf tahor." In the second, the arrangement is reversed, a permitted bird brooding over "beitzei ohf tamei," eggs produced by a forbidden species. For each case the mishnah rules "patur mishale'ach": no sending away is called for.
The mitzvah is generated only when the mother and the brood belong together and both are kosher. A kosher clutch of eggs alone does not create the obligation if the bird sitting on them is not kosher. And a kosher bird sitting on eggs that are not kosher is likewise exempt.
The second half of that requirement is learned from the continuation of the verse: "ve'et habanim tikach lach," and the young you shall take for yourself. The young have to be something you yourself can eat. A brood that could only be thrown to your dogs is not "for yourself." So the mitzvah of shiluach haken takes effect only where the eggs or chicks are fit for your own consumption. Where they are not, even a kosher mother bird sitting over them need not be sent away.
The Male Partridge
The closing case of the mishnah is "koreh zachar," the male of the partridge. This species may be eaten, and what makes it noteworthy here is that its males take their turn incubating the clutch no differently than the females do.
Rebbi Eliezer rules "mechayev": the bird has to be driven off before anything is taken. Since the male in this species carries out precisely the task a mother bird performs elsewhere, Rebbi Eliezer counts him as the brooding parent the Torah spoke about, his being male notwithstanding.
"Vachachamim potrim," the Chachamim exempt him. In their view the mitzvah of shiluach haken was given with regard to a female bird alone, and no set of circumstances can extend it to a male.