Spirit Terrorbird scrolls restore more energy if you have a higher agility level :). Stated, it's FLOOR((Agility level)/2), or your agility level divided by 2, rounded down. So while someone with 60 agility will get 30 energy per scroll, someone with 99 agility will get 49 energy per scroll. This is an added benefit of having a higher level agility, and makes training it worthwhile (well, maybe not to level 99, but to a higher level anyway).