Quotation zu Megillah 4:11

The same reason applies to the law which enjoins that we should let the mother fly away when we take the young. The eggs over which the bird sits, and the young that are in need of their mother, are generally unfit for food, and when the mother is sent away she does not see the taking of her young ones, and does not feel any pain. In most cases, however, this commandment will cause man to leave the whole nest untouched, because [the young or the eggs], which he is allowed to take, are, as a rule, unfit for food.