Before the ELH ALL ecls are at least equal to the number of HD.
The low Str more than compensates for the flying and the small size. Add in the lack of hands, and the difficulties with equipment and you wind up with an ECL lower than 2, so it's +2 (2 HD creature).
Basically your raven relies on spells to do his fighting for him (no Str, no hands for bows). Playing a Raven seems a silly choice for a combat heavy game. In a roleplaying heavy game it can have some great advantages, though (who would suspect the raven of eavesdropping?).