Chicken cant be killed on the id 31:1

I was playing minecraft and was trying to kill a chicken and we all know
sword's cant destroy Blocks
the bug is if a chicken goes into a grass not the grass block but the grass with the id 31:1 and the chicken have to be in the middle of the block if not can i kill the chicken sorry if im typing wrong im Danish not English i have made i sword the is called WorldsBestSword and its enchaned very good and im playing on bukkit and i have also tryed it in a singeplayer the chicken cant be killed