Mike Keesey wrote:If you use it for the crown group, then it's definitely not a bird, but there are many Mesozoic non-birds smaller than _Microraptor_.
Let me take a moment to endorse this particular definition. It is problematic to to define Archaeopteryx into birds a priori and doing so has repeatedly resulted in people extrapolating derived crown-group features and behavior back to all "birds".
-- Palaeontography: http://palaeo.jconway.co.uk