Our source is the Bible, and the Bible has absolute authority. We know there were discreet Kinds because it says so. But it doesn't define them. That's for science to do.
But does the Bible even give any clues as to what a kind might be? It may not define "kind", but are there examples? What does the Bible say?
According to the Flood story, animals were put on the Ark by "kinds" in groups of 2 and 7, right? The only specifically identified animals are the dove and the raven (as far as I recall these are the only ones). Therefore, by definition, these two birds would have to be representative "kinds" of the animals that would have made it onto the Ark.
So if this is any indication, "kind" would appear to be somewhere about the genus level of classification. So if we go by the Bible example of a "kind" being a raven and a dove, it appears to be more specific than just "bird-kind" and similar groups.
~clpMINI