Had Buddha really seen his incarnations before he was a human ? He claimed to be many species like insects, birds, animals, etc. before he became a human. According to the human cloning book it is possible, but how is it possible to upload an advanced mind (a human or sentient mind) in a non sentient animal like a bird (considering that a bird has a very low consciousness compared to a higher being).

I don't know if Buddha saw his incarnations, or if he truly said such a thing. It could very well be a misinterpretation of an ancient scribe.

Regarding uploading a human mind onto a bird or another animal, I suppose that there are several ways to do it. One way could be a full immersion in a virtual world, so the capacity of the brain is not constrained by the size of the animal.

Another way, I imagine, could be to have the minimum necessary parts of the human brain uploaded into an animal, so that the person can feel what an animal feels. I assume that some higher functions such as creativity, math, language... and large chunks of our memory... might not be included in the upload.

From my own observation with pets, animals are very much like humans, except that they don't have the ability to question or modify their environment. At the same time, they are very basic in the sense that all their actions are aimed at survival (self-preservation, food and procreation). They also play and have relationships, but at quite a basic level...