Deuteronomy 22:6

Thou shalt not take, &c. This was to shew them to exercise a certain mercy even to irrational creatures; and by that means to train them up to a horror of cruelty; and to the exercise of humanity, and mutual charity one to another. (Challoner) --- Some were of opinion that the person who could take the old bird on the nest might assure himself of good fortune, fecundity, &c. (St. Thomas Aquinas, [Summa Theologiae] i. 2, q. 102, a. 6.) Such superstition is reprehensible. Phocilides advises not to take all the young ones, nor the hen, in consideration of one's having more birds. (Calmet)