Not only is there a prohibition against taking a mother bird along with her young (as we saw in the previous mitzvah), there is an obligation to send the mother bird away.
The reason for this mitzvah… ay, there’s the rub. The easy answer is that this is a sign of the providence that God extends over all His creatures. Rambam tells us that the mother would suffer if she had to watch her chicks being taken. Nevertheless, we are left with something of a dilemma to resolve. You see, the Talmud tells us (Brachos 33b) that if the leader of a prayer service should ad-lib that God’s mercy extends even to the bird’s nest, he is quickly stopped. The reason for this is that the mitzvah is because God told us to do so, and who are we to try and ascribe motivations to His commands? And yet, here we are, ascribing motivations to His commands – and we have been for 545 mitzvos!
The Rambam resolves this apparent difficulty by telling us that there are two Talmudic opinions. One is that all mitzvos are to be seen as Divine decrees and we should not presume to ascribe motivations to them. The other opinion permits us to analyze the mitzvos, learn lessons and infer reasons. The Rambam, the Sefer HaChinuch and we here have all been following this second approach.
Despite this, I would not recommend ad-libbing about God’s mercy and the bird’s nest in prayer. In reality, the mitzvos don’t do anything for God, Who lacks nothing; they’re for us. So we can’t really say that the reason for this mitzvah is that God is merciful (even though He certainly is). What we can say is that through this mitzvah, God teaches US to be merciful.
This mitzvah only applies to wild birds in a nest one might encounter in the public domain. If a farmer has a chicken coop, for example, he does not send the hens away whenever he gathers their eggs.
This mitzvah promises one a long life (Deut. 22:7). Another mitzvah that promises a long life is honoring one’s parents (Exodus 20:12). The sage Elisha ben Abuyah became a heretic after he saw a father instruct his son to get some eggs from a tree. The boy climbed the tree, sent away the mother bird, fell down and died. Here, the boy had fulfilled both the obligation to honor his father and to shoo away the bird, yet he had not received a long life! The fallacy in Elisha ben Abuyah’s analysis is that the phrase “long life” does not mean physically. Its referent is spiritual, i.e., in the Next World. (See Talmud Kiddushin 39b; Rav Yaakov analyzed the same incident properly.)
You will note that honoring one’s parents can be very difficult to do but shooing away a bird is pretty simple – yet they offer the same reward! From here we see that we cannot guess the relative “value” of mitzvos. What we consider to be a relatively unimportant matter might be quite significant indeed!
This mitzvah applies in all times and places. It is discussed in the Talmud in tractate Chullin starting on page 138b. It is codified in the Shulchan Aruch in Yoreh Deah 292. This mitzvah is #148 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos and #74 of the 77 positive mitzvos that can be fulfilled today as listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.