Deutéronome 22:6

כי יקרא: פרט למזומן: If a bird’s nest chances before you: This excludes [a bird nest that is] ready at hand. - [Chul. 139a, Sifrei 22:55]

you shall not take the mother: while she is on her young, [whereas if she is only hovering overhead, you may take her from upon her young]. - [Chul. 140b]

If thou find the nest of a clean bird before thee in the way, in a tree, or upon the ground, in which there are young ones or eggs, and the mother sitting upon the young ones or eggs,

The passage of if you happen upon is tied to an earlier passage through the common phrase “on the road” [: 4].

happen upon [Hebrew: yiqqare ] from the word “happenstance” [Hebrew: miqreh] [I Samuel 6: 9] [albeit here the word is spelled with an ’alef — Translator] (as in “as I happened by chance” [II Samuel 1: 6]). Here it means “to encounter”.

The ’alef in the word young ones is extraneous, like the ’alef in “and with your [actually, “an” — Translator] arm” [Jeremiah 32: 21]. The point of this commandment is that it would be profoundly cruel if “the mother is dashed in pieces upon her children” [Hosea 10: 14], just as in the case of an ox, or a lamb [Leviticus 22: 28].

IF A BIRD’S NEST CHANCE TO BE BEFORE THEE. This also is an explanatory commandment, of the prohibition ye shall not kill it [the dam] and its young both in one day, because the reason for both [commandments] is that we should not have a cruel heart and be discompassionate, or it may be that Scripture does not permit us to destroy a species altogether, although it permits slaughter [for food] within that group. Now, he who kills the dam and the young in one day or takes them when they are free to fly [it is regarded] as though he cut off that species.
Now, he [Rabbi Moshe ben Maimon] wrote in the Moreh Nebuchim that the reason for the commandment to release the mother bird when taking its nest and the prohibition against killing the dam with its young on one day is in order to admonish us against killing the young within the mother’s sight, for animals feel great distress under such circumstances. There is no difference between the distress of man and the distress of animals for their young, since the love of the mother and her tenderness to the children of her womb are not the result of reasoning or [the faculty of intelligent] speech, but are produced by the faculty of mental images So in Al Charizi’s translation which was used by Ramban. In Ibn Tibbon’s translation the term is ham’dameh — “imagination.” which exists among animals even as it is present in man. But if so the main prohibition in killing the dam and its young applies only when killing [first] the young and [then] the dam [but not vice versa, whereas the Torah forbids it to be done either way]! But it is all an extraordinary precaution, and it is more correct [to explain them as prohibitions] to prevent us from acting cruelly.
And the Rabbi [Moshe ben Maimon] said further: “Do not contradict me by quoting the saying of the Sages, ‘He who says in his prayer: Even to a bird’s nest do Thy mercies extend [etc., they silence him,’ which would seem to imply that there is no reason other than the Will of G-d for the commandment to release a dam when taking its nest], for that is one of two opinions, namely, the opinion of the Sage who holds that the commandments [of the Torah] have no other reason but the Will of the Creator. We follow the second opinion that there is a reason for all commandments.” And the Rabbi [Moshe ben Maimon] raised a difficulty from a text in Bereshith Rabbah [which contradicts his theory that there is a reason for every commandment]. …

כי יקרא קן צפור, when one chances across this without having planned it.

לא תקח האם על הבנים, I have already elaborated on this verse in connection with the prohibition to boil the young in the milk of its mother (Exodus 23,19) Similar considerations are valid in connection with the prohibition of אותו ואת בנו. The practices, if carried out, look as if the person carrying it out is insensitive, cruel by nature, and is so anxious to satisfy his palate that all considerations of feelings of others are of no concern to him.

כי יקרא כן צפור לפניך בדרך, “if you encounter by chance a bird’s nest on your way, etc.;” the reason why these paragraphs follow one another is that the performance of one commandment brings in its wake the performance of additional commandments. (Compare Devarim Rabbah, 6,4 on our portion). Seeing that the reward for the performance of the commandment just discussed results in your enjoying long life, you will have the opportunity to devote yourself for more years to Torah study. When you build a house for yourself, you will be able to perform the commandment of erecting a fence on the edges of its roof to protect anyone from falling off it. When the Torah follows with the warning not to mix seeds of different plants when planting a vineyard, you will enjoy harvesting the fruit of that vineyard. When you are warned not to harness an ox and a donkey to the same plough, [as it would result in the donkey suffering pain, seeing it is so much weaker. Ed.] you will enjoy owning many oxen and many donkeys as a result of observing that commandment. When the Torah subsequently warns you not to wear garments in which linen and wool have been woven in the same cloth [except when commanded to do so for the tzitzit in certain situations], you will be able to afford buying a beautiful tallit on which to perform the commandment to attach these fringes. (Compare Tanchuma, section 2). The commandment to own garments which qualify for the attaching of these fringes is extremely important as we know from the Talmud, tractate Shabbat folio 32. According to a statement on that folio, anyone who is meticulous about the observance of that commandment will eventually be in command of 2800 servants. This statement is based on Zecharyah 8,23: בימים ההמה אשר יחזיקו עשרה אנשים מכל לשונות הגוים והחזיקו בכנף איש יהודי לאמר נלכה עמכם כי שמענו אלוקים עמכם, “in those days ten men from nations of every tongue will take hold,-they will take hold of every Jew by a corner of his cloak and say: ‘let us go with you, for we have heard that G–d is with you.’ [There are 70 tongues, multiplied by ten multiplied by four corners on each garment of a Jew adorned with tzitzit. Ed.] This is also the interpretation of Jeremiah 17,11-12: קורא דגר ולא ילד, עושה עושר ולא במשפט, “like a partridge announcing that it has hatched young when this is not true, or like amassing riches illegally;” this is followed by the line: “כסא כבוד מרום מראשון, “o throne of glory exalted from of old;” how does this line connect with the line preceding it?” it teaches that the partridge proclaiming to have hatched young from its eggs had stolen those eggs from another species of bird sitting on them only after they had already been hatched. Then they proceed to eat those chicks which belong to another category of bird. When the chicks try to fly and are unable to, they become victims to predators. Who caused this sequence? Whoever steals eggs that are not his own. The prophet uses this simile to describe the gentiles who boast about their ill gotten gains which are the result of robbery. These gentiles, who destroyed the Holy Temple by burning it, will eventually have to pay for their crime by becoming totally extinct. (Compare Tanchuma, section two on our portion)