A mitzvah drags another mitzvah with it
כי יקרא קן-צפור וגו’ והאם רבצת על-האפרחים או על-הביצים לא-תקח האם על-בנים: שלח תשלח את-האם ואת-הבנים תקח-לך למען ייטב לך והארכת ימים: כי תבנה בית חדש ועשית מעקה לגגך ולא-תשים דמים בביתך כי-יפל הנפל ממנו: לא-תזרע כרמך כלאים וגו’ לא-תחרש בשור-ובחמר יחדו: לא תלבש שעטנז צמר ופשתים יחדו
When you chance upon a bird’s nest…and the mother bird is crouched on the chicks or on the eggs, don’t take the mother bird [with]2 the children. [Rather], send away the mother bird, and take the children for yourself. This is so it will be good for you and it will lengthen your days. When you build a new house, make a fence for your roof. Don’t place blood in your house, since a person will fall from [the roof without one]. Don’t sow your vineyard with mixed crops…don’t plow [your field] with an ox and donkey together. Don’t wear sha’atnez, [which is] wool and linen together
This week’s parsha contains within it more mitzvos than any other, totaling seventy-three. Sometimes it’s easy to understand why the Torah grouped certain mitzvos together, and other times not as much. There are a series of mitzvos that describe forbidden mixtures in this week’s parsha, and they are understandably grouped together. There is a prohibition on sowing mixed crops together in the same vineyard. There is a prohibition against doing field work with two different animals together. There is a prohibition for our garments to be made of a mixture of wool and linen. However, the mitzvos that precede these mixture-mitzvos seemingly have no connection to what follows them. First, the Torah describes how to interact with a mother bird and her children. If the passerby wants the chicks, they have to first send away the mother bird. Subsequently, the Torah commands building a fence on our roof when we get a new house. This will prevent any mishaps from occurring. Afterwards is the above-mentioned mixture-mitzvos. What can we learn from this confusing juxtaposition?
Rashi tells us that there is a subtle message being conveyed. If you fulfill the mitzvah of sending a way the mother bird, you will consequently build a new house, and fulfill the mitzvah of building a fence on its roof. This is because מצוה גוררת מצוה, literally: a mitzvah drags another mitzvah with it. This means that fulfilling one mitzvah makes one prone to fulfilling another. Afterwards, a person will acquire a vineyard, a field, as well as nice garments. This is why these mitzvos are juxtaposed to each other. To clarify, what’s clear from Rashi’s words is that these acquisitions aren’t provided as rewards for our mitzvah fulfillment. Rather, the mitzvah that accompanies them is the reward.
The new house that a person builds isn’t itself a reward. It’s really a tool to provide the opportunity to fulfill a new mitzvah. Since it’s impossible to fulfill the mitzvah of building a fence on the roof without a new house, Hashem provides the person with a new house. This was because they fulfilled the mitzvah of sending away the mother bird. So too with the new vineyard. It was provided to the person as a reward for building a new fence, specifically so they get the opportunity to not sow mixed crops. As well, by not planting mixed crops, the person is given a field. This is so they can fulfill the mitzvah of not working with different animals together. Finally, by not working with their animals together, the person is provided with clothes that they can ensure don’t contain sha’atnez.
In addition, what we see from this is that if a person acquires a new house and doesn’t fulfill the mitzvah of building a fence on their roof, isn’t guaranteed to keep it. It was only given to the person as an avenue to fulfill the mitzvah. Who says they can keep it even if they don’t fulfill the mitzvah? It wasn’t a reward for sending away the mother bird; the mitzvah of building the fence was his reward. It was only a means to an end. Without the end, the means is taken away.
With this explanation comes a new understanding of what Targum Onkelos means when he translates “fence” as תיקא. Rashi explains that it’s the same word as תיק, or pouch. Meaning, build your fence like a pouch, which protects that which is inside it. Rashi is saying that he simply means prevent the people in the house from falling off the roof by building a protective fence. However, according to the above explanation for the juxtaposition of the verses, we can say a different intent with the analogy of a pouch. Obviously the fence is protecting the people in the house, but it’s also protecting something else. The mitzvah of building a fence protects the person from losing their house. The whole reason Hashem gave the person the house was so they would fulfill the mitzvah. By following through, they are protecting the gift that Hashem gave them from being taken away. This is like a pouch, which protects that which is inside it.
 Based on Be’er Yosef to Deuteronomy 22:8
 Deuteronomy 22:6-11
 Some say even if you don’t want the eggs. See note 2
 However, the following explanation doesn’t address why the Torah is specifically in this order. Any mitzvah should provide the avenue for any other mitzvah. It shouldn’t follow that specifically sending away the mother bird provides the avenue for specifically the mitzvah of building a new fence
 to verse 8, quoting Midrash Tanchuma Ki Seitzei § 1
 Avos 4:2
 See Nefesh HaChaim 1:6 and Sifsei Chaim Moadim I Elul, עבר ושנה – הותרה לו for more on this idea
 The Be’er Yosef doesn’t address why Rashi specifically said nice clothes
 verse 8
 ad. loc. s.v. מעקה