Flying is already a very demanding and difficult task - how much larger a brain could their metabolism support?
We don't know that a larger brain is required for greater intelligence (in e.g. birds).
I agree. But it's "easier" to evolve a slightly larger brain with the same architecture, than to discover a new, more efficient architecture.
In general we should expect a larger brain (ours consumes 1/4 of our total energy) to pay for itself in more actual intelligence; if it doesn't (the architecture can't easily scale) then the smallest (still working) brains will win.
For each species of bird, either
1) general scalable intelligence isn't supported by their brain's architecture (so you can't just grow more processing power)
2) birds lack the ...
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.