Which to me raises a question: how much smarter could birds get? Flying is already a very demanding and difficult task - how much larger a brain could their metabolism support?
I suspect that ravens and parrots are close to this limit, and that higher-calorie birds would have to be flightless. Penguins and other flightless birds like the ostrich are, IIRC, the heaviest and largest birds there are.
This present a problem for any intelligent-bird lineage: the groups that have demonstrated a need for intelligence (like the parrots and the ravens) are very separate from the groups that have demonstrated their access to the calories/protein necessary for human-level intelligence. So how they get here from there?
I've always wondered how intelligent dogs, birds, chimpanzees, bonobos, dolphins, rats, etc. could become with say, 100 years of rigorous scientific selective breeding for intelligence.
I can't imagine any of them would reach human level intelligence (unless they had some really lucky mutations), but they might become extremely interesting, and highly instructive about the nature of intelligence.
We have a sample of one modern human civilization, but there are some hints on how likely it was to happen.
Major types of hints are:
Data for:
Data against:
To me it looks like life, animals with nervous systems, Upper Paleolithic-style Homo, language, and behavioral modernity were all extremely unlikely events (notice how far ago they are - vaguely ~3.5bln, ~600mln, ~3mln, ~200k or ~600k, ~50k years ago) - except perhaps language and behavioral modernity might have been linked with each other, if language was relatively late (Homo sapiens only) and behavioral modernity more gradual (and its apparent suddenness is an artifact). Once we have behavioral modernity, modern civilization seems almost inevitable. Your interpretation might vary of course, but at least now you have a lot of data to argue for your position, in convenient format.