Parashat Ki Tetze contains the commandment of shiluah ha-qen, where the Torah states the following:

If, along the road, you chance upon a bird's nest, in any tree or on the ground, with fledglings or eggs and the mother sitting over the fledglings or on the eggs, do not take the mother together with the young. Let the mother go and take only the young, in order that you may fare well and have a long life (Deut. 22:6-7).

In his work The Guide of the Perplexed, (III:48), Rambam presents his suggestion regarding the reason for the commandment, which is, in his view, also the reason for the prohibition to slaughter an animal and its child on the same day (Leviticus 22:28). Rambam writes as follows:

It is likewise forbidden to slaughter it and its young on the same day (Lev. 22:28), this being a precautionary measure in order to avoid slaughtering the young animal in front of its mother. For in these cases animals feel very great pain, there being no difference regarding this pain between man and the other animals. For the love and the tenderness of a mother for her child is not consequent upon reason, but upon the activity of the imaginative faculty, which is found in most animals just as it is found in man. This law applies in particular to ox and lamb (Lev. ad loc), because these are the domestic animals that we are allowed to eat and that in most cases it is usual to eat, in their case the mother can be differentiated from her young.
This is also the reason for the commandment to let (the mother) go from the nest (Deut. 22:6-7). For in general, the eggs over which the bird has sat and the young that need their mother are not fit to be eaten. If then the mother is let go and escapes of her own accord, she will not be pained by seeing that the young are taken away. In most cases, this will lead to people leaving everything alone, for what may be taken is in most cases not fit to be eaten. If the Law takes into consideration the pains of the soul in the case of beast and birds, what will be the case with regard to the individuals of the human species as a whole? You must not allege as an objection against me the dictum of (the Sages), may their memory be blessed (Mishnah, Berakhot V:3) He who says: Your mercy extends to young birds, and so on.(Such an individual is blamed in the Mishnah). For this is one of the two opinions mentioned by us- I mean the opinion of those who think that there is no reason for the Law except the will (of God) - but for us, we follow the second opinion. (Guide, III:48 [Pines ed.], pp. 599-600)

Ramban, in his comments to Deut. 22:6-7, takes strong exception to the Rambam's approach regarding ta'amei ha-mitzvoth in general (which will not be dealt with here) as well as the specific issue of shiluah ha-qen. Ramban's interpretation of the mitzvah of shiluah ha-qen is as follows:

So too, what the Rabbis have stated (Berakhot 33b), "Because he treats the ordinances of God like expressions of mercy, whereas they are decrees" (and that is why the Mishnah states that He who says: Your mercy extends to young birds is silenced) means to say that it was not a matter of God's mercy extending to the bird's nest or the dam and its young, since His mercies did not extend so far into animal life as to prevent us from accomplishing our needs with them, for, if so, He would have forbidden (animal) slaughter altogether. But the reason for the prohibition (against taking the dam with its nest, or against killing the dam with its young in one day) is to teach us the trait of compassion and that we should not be cruel, for cruelty proliferates in man's soul as it is known that butchers, those who slaughter large oxen and donkeys are men of blood (Psalms 55:24); they that slaughter men (Hosea 13:2), are extremely cruel. It is on account of this (cruelty) that the Rabbis have said (Kiddushin 82a):"The most seemly among butchers is a partner of Amalek." Thus these commandments with respect to cattle and fowl are not as a result of compassion upon them, but are decrees upon us to guide us and to teach us traits of good character. (Ramban, Commentary on the Torah: Deuteronomy, edited and annotated by Rabbi Dr. Charles Chavel [New York, 1976], p. 271)

There seems, then, to be a wide discrepancy between the Rambam and the Ramban. According to the former, shiluah ha-qen and oto ve-et beno are indeed as a result of compassion upon them. One is sorely tempted to use current terminology and say "according to the Rambam, it is a din in the animal; whereas according to the Ramban, it is a din in the human being."
Yet Don Isaac Abravanel did not seem to have understood the dispute between the Rambam and the Ramban in such a fashion. In his discussion of the topic, after quoting the Rambam and the Ramban regarding shiluah ha-qen (and before presenting his own explanation for the mitzvah), he writes:

But I say that his (Ramban's) words are entailed in those of the Moreh (Rambam) and that he (Ramban) did not add anything in this matter. (Perush Ha-Torah le-Rabbenu Yitzhak Abravanel, ed. Schottland [Jerusalem, 1999], p. 343).

How was Don Isaac Abravanel able to fit the Rambam and Ramban so seamlessly together?

I think that two more considerations have to be included to obtain a full picture of Rambam's position, and these issues may have led Don Isaac Abravanel to claim that Ramban essentially was saying the same thing as the Rambam. First, there is the philosophical point that, apparently, had been raised by R. Josef Rosen, the Rogotchover Gaon, z"l. It takes as its point of departure the assumption that the interpretation of Rambam's view in the Guide regarding shiluah ha-qen should cohere with his general understanding in the Guide of divine providence upon the members of the animal kingdom. And on this score, the Rambam's view is clear.

For I for one believe that in this lowly world- I mean that which is beneath the sphere of the moon-divine providence watches only over the individuals belonging to the human species and that in this species alone all the circumstances of the individuals and the good and evil that befall them are consequent upon the deserts, just as it says: For all his ways are judgment (Deut. 32:4) But regarding all the other animals and, all the more, the plants and other things, my opinion is that of Aristotle. For I do not by any means believe that this particular leaf has fallen because of a providence watching over it; not this spider has devoured this fly because God has now decreed and willed something concerning individuals; nor that the spittle spat by Zayd has moved till it came down in one particular place upon a gnat and killed it by a divine decree and judgment; not that this fish snatched this worm from the face of the water, this happened in virtue of a divine volition concerning individuals. For all this is in my opinion due to pure chance, just as Aristotle holds. (Guide, III:17, p. 471)

Thus, Rambam's view of Providence seems to lead him to agree that "it was not a matter of God's mercy extending to the bird's nest or the dam and its young, since His mercies did not extend so far into animal life," as the Ramban says.

Moreover, there is the matter of Rambam's view regarding tza'ar ba'ale hayyim. (Whether his view in Mishneh Torah is that the prohibition is min ha-Torah or mi-derabanan is a thorny issue that is beyond our purview here.) Suffice it to say that in the Guide, Rambam's words regarding shiluah ha-qen and oto ve-et beno clearly imply some notion of tza'ar ba'ale hayyim min-ha-Torah. That is why one cannot let an individual animal receive psychic pain (and certainly not physical pain).Yet how can that be? If God's mercies are not extended to individual animals should human mercies exceed those of God? (I am indebted to the late Rabbi Josef Wanefsky of Yeshiva University, who shared with me a latter he received from the late Rabbi Schneider [of Monticello, N.Y.] on the topic, for first bringing this issue to my attention.)
Apparently (and this may be the assumption of Don Isaac Abravanel), even the Rambam essentially maintains that the focus is essentially in developing the human sense of compassion. According to this reading, the key passage is the qal ve-homer that concludes the Rambam's presentation above.
If the Law takes into consideration the pains of the soul in the case of beast and birds, what will be the case with regard to the individuals of the human species as a whole?

If one insists on maintaining the yeshiva terminology used above, one may say that according to the Rambam, shiluah ha-qen and oto ve-et beno are indeed mitzvoth that are "dinim in" the person,. The heftzah shel mitzvah, on the other hand, the object that helps one develop as a sensitive human being, is the animal that is suffering.

The musar haskel is clear. One must treat all of God's creatures with compassion. Every particular time that one does so, one elevates himself. As the Rambam subsequently writes:

For it is by all the particulars of the actions and through their repetition that some excellent men obtain such training that they achieve human perfection, so that they fear, and are in dread and in awe of, God, may He be exalted, and know who it is that is with them and as a result act subsequently as they ought to. (Guide III:52, pp. 629-30