Sending Away the Mother Bird

There is an interesting machlokes acharonim regarding the mitzvah of Shiluach haKan. If one happens upon a bird's nest and does not need the eggs - must he send away the mother bird anyway?

The Kollel Iyun HaDaf discusses this issue (link - lightly edited for clarity purposes):

The Pischei Teshuva (YD 292:1) cites the Chavos Yair (#67) who concludes that one is obligated to send away the mother bird whenever possible. He proves this from the Gemara here [Chullin 139b], which he understands to be saying that one is not obligated to go searching for a bird's nest in order to fulfill the Mitzvah, but one is obligated to perform the Mitzvah when he chances upon a bird's nest, even if he does not need the eggs. (The Chavos Ya'ir cites proof for this ruling from the words of the Zohar; see Insights to Chulin 138:5.)

This is also the view of the Maharal (Tiferes Yisrael, end of chapter 61), Maharsham (1:209), Birkei Yosef (YD 292:8), and Aruch HaShulchan (YD 292:1-2).

This obligation applies even to a person who has absolutely no interest in owning the contents of the nest, and even if stopping to fulfill the Mitzvah will cause him to suffer a monetary loss, as the Chasam Sofer (OC #100) and Netziv (in Meromei Sadeh here) explain this view. The reason for this is that since the performance of this Mitzvah hastens the Ge'ulah (as described in Insights to Chulin 138:4-5), one is not allowed to squander such an opportunity, and thus it is a Halachic requirement to fulfill the Mitzvah. In addition, a person who sees a nest and does not perform the Mitzvah is punished in a time of Divine anger (Pischei Teshuvah loc. cit.).
It is interesting to note that the Arizal (quoted by Rav Chaim Vital in his introduction to Sha'ar ha'Mitzvos; Birkei Yosef, Gilyon Shulchan Aruch YD 292:6, and Aruch ha'Shulchan YD 292:1) writes that according to Kabbalah, one must make every effort to perform Shilu'ach ha'Ken. He adds that one who does not perform the Mitzvah of Shilu'ach ha'Ken will return to this world as a Gilgul.)

However, the Chacham Tzvi (#83) and Chasam Sofer (OC #100) rule that when one has no need for the offspring, he is not obligated to send away the mother bird. The Chasam Sofer adds that if the purpose of the Mitzvah is to inculcate in us the trait of compassion (see Insights to Chulin 138:4), then it is clear that we are not obligated to send away the mother bird when we have no need for the offspring, because doing so causes distress to the bird for no reason.

This is also the opinion of many Rishonim, including Tosafos (140b, DH Shnei), the Rambam(Hilchos Shechitah 13:5), Ran, Meiri (139b), and Rabbeinu Bachya (end of Devarim 22:7).
This view agrees that it is meritorious and commendable to pursue and perform the mitzvah, but it is not mandatory to do so and one is not punished for not doing so. In addition, all opinions agree that the Mitzvah is fulfilled l'Chatchilah even when one desires the contents of the nest exclusively so that he can send away the mother and perform the Mitzvah, and he will not use the contents afterwards.

Most Acharonim rule that there is no obligation to send away the mother bird when one chances upon a nest and has no need for the eggs. This is the ruling of the Chasam Sofer (loc. cit), Avnei Nezer (OC #481), Chazon Ish (YD 175:2), Chazon Yechezkel, Minchas Chinuch (#544), and the Chafetz Chaim (in Sefer HaMitzvos HaKatzer, Mitzvos Aseh #74). Most contemporary Poskim also rule this way, including Rav Shlomo Zalman Auerbach zt'l (in Minchas Shlomo 2:5:4), and Rav Yosef Shalom Elyashiv shlit'a and Rav Chaim Kanievsky shlit'a (in personal conversations with Rabbi Naftali Weinberger). This common practice today. (Rabbi Weinberger quotes Rav Yakov Yisrael Fisher zt'l, however, who was of the opinion that one is obligated to send away the mother bird when he chances upon a nest, even though he does not need the eggs).

Thus, although many acharonim say there is no obligation to send away the mother bird if you don't want the eggs, a significant number argue and say that the obligation is there in any case - as long as you happen upon a bird's nest.