I'll give you the tip a friend gave me for the time I tried this:
"You can have 100% chance but if you only have 140aspd it wont matter, where-as 190apsd with 20% will deal more dps and trigger the bird easily"
Or something along those lines. You're good with around 15~20% if you have a nice aspd =P, also remember, bird hits with neutral element so your over all dmg will still come from your arrows.