R. Yossi Haglili says: It is written "You shall not eat any carcass," and "You shall not cook a kid in the milk of its mother." What is forbidden as neveilah is forbidden to cook in milk. A bird, which is forbidden as neveilah, I might think that it is forbidden to cook it in milk; it is, therefore, written "in its mother's milk" — to exclude a bird, which has no mother's milk.
Variantly: "You shall not cook a kid in its mother's milk": Why is this written three times? It corresponds to the three covenants that the Holy One Blessed be He made with Israel: one in Chorev, another in Arvoth Moav, and another in Mount Gerizim and Mount Eival.