whatever those birds origin was, the bottomline is that they came by Allah's command (whether they were worldly birds or came out of nowhere, etc) and they carried stones which they threw on the army of abraha. these were no ordinary stones. I heard that the stones would enter the flesh and would liquify the flesh.
It is not right to discuss things like where the birds were from,, were they of this world or of hell, and overlook the main thing, which is the punishment of Allah. In such discussions the main thing is forgotten.