The gall of these scientists to claim that not only can they definitively say what consciousness is, but that they can prove that something has it is astonishing.
That isn't what they say. The author of the article is editorializing, and the HN title needs to be changed. (puts up mod alert spotlight).
What they say is what consciousness is not, that is, it does not require a complex brain, that's not a prerequisite. They don't say very much about what it is, just its prerequisites. Buried in their supporting arguments, they state that "birds appear to offer" "parallel evolution" of "near-human-like levels of consciousness". As a non-biologist I am not entirely sure what exactly is meant by that, but it is not within a hundred miles of a "proof".