According to Smogon, the central for research on how Pokemon works, it halves all stats by 50%. Other sources say just Attack, some say all stats, at one point, Smogon said just Attack and Speed.
NOTE: I've Just found this. Since the Research Thread on Smogon now reads confirmation on all stats, I'll assume that. Regardless, it's a crappy ability, and this sick-looking bird has no hope in the Metagame.