Chullin, Chapter 12, Mishnah 1. The closing perek of the masechet is given over to the mitzvah of shiluach haken. The Torah frames this as a pair of commands that function together: a positive one, obligating a person to drive the mother off the nest before he removes what lies in it, and a negative one, forbidding him to seize her while she rests over her fledglings or her eggs. As the perek unfolds we will see how the two of them interact.
Before entering the mishnah itself, there is a well known machloket among the acharonim regarding when this mitzvah is triggered at all. One approach holds that the obligation is unconditional: a person who comes upon a nest is bound by the mitzvah even if he has no interest whatsoever in the eggs or the chicks. He should send away the mother and take the young regardless. The second approach ties the mitzvah to a purpose. If a person has no desire for what is in the nest, there is nothing for the sending away to accomplish, and no mitzvah arises. On this view, the obligation enters the picture only when someone actually wants to take the eggs or the chicks.
The Scope of the Mitzvah
The mishnah begins: "Shiluach haken noheg ba'aretz uvechutzah la'aretz", this obligation is binding inside Eretz Yisrael and beyond its borders alike. It does not belong to the category of mitzvot tied to the soil of the land. "Bifnei habayit veshelo bifnei habayit", it holds whether or not the Beit Hamikdash is standing, since nothing about it depends on the avodah carried out there.
"Bechullin aval lo bemukdashin", the obligation operates with respect to ordinary unconsecrated birds, and not with respect to a bird that has become hekdesh. The pasuk's own phrasing shows why: "shaleiach teshalach", you shall certainly let her go. A consecrated bird cannot be let go; it is the property of the Beit Hamikdash and must be brought there. Where release is not a possibility at all, the mitzvah has nothing to attach to.
Comparing Shiluach Haken to Kisui Hadam
The mishnah now places our mitzvah beside a different mitzvah involving birds: kisui hadam, covering the blood after shechitah. "Chomer bekisui hadam mishiluach haken": in certain respects kisui hadam is the broader of the two, reaching cases where our mitzvah does not.
"Shekisui hadam noheg bechayah uve'of", covering the blood applies both to a wild animal, a chayah, and to a bird. "Bimzuman uveshe'eino mezuman", and it applies whether the creature is mezuman, readily at hand, meaning it is kept in one's home, or eino mezuman, not at hand, living out in the wild.
"Veshiluach haken eino noheg ela be'of", our mitzvah is confined to birds and does not extend to other undomesticated creatures. "Ve'eino noheg ela beshe'eino mezuman": within the category of birds it is narrowed further, applying only to those not readily accessible, that is, birds not residing inside a person's house. The derivation comes from the phrasing of the pasuk: "Ki yikarei ken tzipor lefanecha", should a bird's nest chance upon you. The verb suggests a chance encounter, a nest that comes into view unexpectedly, rather than one already situated in one's own domain and under one's control.
What Counts as Not Readily Available
The mishnah supplies its own definition. "Eizehu she'eino mezuman?" What is an example of a bird that is not readily available, where shiluach haken does apply? "Kegon avazim vetarnegolim shekinnenu bapardes", geese and chickens that have left their owner's home and built their nest out in the orchard.
The chiddush here is significant. These birds were once owned and domesticated, yet since they are no longer inside the house and are free to fly off as they please, they are no longer in their owner's practical grasp. They carry the status of eino mezuman, and the mitzvah of shiluach haken governs them fully.
A kal vachomer follows immediately. If creatures that once had an owner fall under the mitzvah, then birds that never belonged to anybody, the wild ones a person comes across while traveling or out among the trees, are certainly included, and the requirement to release the mother clearly governs them.
"Aval im kinnenu babayit", but if the birds never left, and are nesting within the house itself, the ruling reverses. "Vechein yonei hardesiyot", and the same holds for Herodian doves. These are named for Herod, who kept doves in his palace; they were raised to dwell among people and do not fly away. Such birds are considered mezuman, fully at hand, and therefore "patur mishiluach", one is exempt from sending the mother away. A person may take the mother if he wishes, or take the young if he wishes. In this case there is no mitzvah of shiluach haken at all.