No one has done a Vedic animal sacrifice for hundreds of years, but the Shakta, Shaiva, and Vamachara traditions, especially in East India, are known to perform lots of avaidik animal sacrifices for Durga and Kali.
My question is, does this practice have any scriptural authority?
It is well accepted in Vedanta that avaidik animal sacrifice is sinful, whereas vedic animal sacrifice is dharmic.
Here are some verses from the Manusmriti that denigrate avaidik animal sacrifice and support vedic animal sacrifice:
At the Madhuparka offering, at sacrifices, and at the rites in honour of the Pitṛs,—at these alone should animals be killed, and nowhere else: thus has Manu declared—(41)
The twice-born person, knowing the real import of the Veda, killing animals on these occasions, carries himself and the animal to the most excellent state.—(42).
That killing which is sanctioned by the Veda has been eternal in this world of mobile and immobile beings: it is to be regarded as no killing at all; since it was out of the Veda that the Law shone forth.—(44).
As many hairs there are on the body of the animal, so many times after dying does its needless killer suffer violent death, birth after birth.—(38.)
Now this verse from the Manusmriti directly condemns avaidik animal sacrific and prescribes an alternative:
If there is occasion, he shall make an animal of clarified butter, or an animal of flour; but he shall never seek to kill an animal needlessly.—(37)
Here is what Medhatithi says in his commentary on that verse:
People are likely to entertain such ideas as the following—“at the Sītā - yajña the Khañjikā-yāga, the Caṇḍikā-yāga and the like, which are performed on the authority of usage only, it is right for the man desiring a certain result to kill animals; for it has been found that one obtains a rich harvest by offering sacrifices at which animals are killed.”...... such sacrifices as those mentioned cannot be assumed to have any sanction in the Veda.
The texts known as the Kalpataru and the Vīramitrodaya also says this:
In such ceremonies as the Sītāyajña and the like, which are not prescribed in the Veda, and the killing of animals at which, therefore, cannot have the sanction of the Veda,—if, in view of the prevalent custom, it is found necessary to sacrifice an animal, one should offer an animal made either of butter or of flour.
Now some people might interpret this Manusmriti verse to mean that animals in Vedic yajnas can be substituted with fake animals made of flour or butter, but the author of the Vīramitrodaya rejects the following explanation on the following grounds:
it would not be right for a Smṛti to lay down an alternative to a detail that has been laid down in the original Vedic injunction of the sacrifices.
So given all this, what authority is there to support the animal sacrifices in the Shaiva and Shakta traditions?