I assume your gun is a 3", if so then stick with the 3/4oz 6s or 5s. (4s just wont have enough pellets IMHO, only 101 in a 3/4oz load. Nearly 170 in 6s!) Keep your shots inside 25-30yds and MAKE SURE she is DEADLY ACCURATE, otherwise your running a REAL risk of her wounding that bird with a shot that were she shooting a 20ga or larger, she would've killed the bird!

If the guns only a 2.5" chamber then I'd seriously have reservations about using it as you'll only find .5oz loads. (Only about 65 pellets in a .5oz load of 4s. In a .5oz load of 6s you'll have about 110 pellets!) What the 2.5" loads lose in pellet count to the 3", it gains in velocity. (2.5" appx 1250fps vs 3" appx 1100fps) If she can put 4-5 pellets in the head/spine then the birds lights WILL be out. But you've GOTTA be honest as to range, guns patterning abilities and most importantly... HER abilities!!!