In Birthright's Rulebook, page 74, about Erik's priesthood is stated:
PW as druid, plus move silently, hide in shadows, and animal empathy as a ranger three levels higher
Since in Birthright there is more than one church of Erik, and the head of the religion is a regent who can be a level 1 npc (or player), and at 15th level (from PH):
The Grand Druid knows six spells of each level (instead of the normal spell progression) and also can cast up to six additional spell levels, either as a single spell or as several spells whose levels total to six (for example, one 6th-level spell, six 1st-level spells, three 2nd-level spells, etc.)
Or it is not considered a druid power, since derives from position, as his servents receive some too:
Three of these nine are archdruids who roam the world, acting as his messengers and agents. Each of them receives four additional spell levels.
If so, hierophants powers are ignored too? Since their origin is from hierophant status and comes at cost of spells:
Beyond 15th level, a druid never gains any new spells (ignore the Priest Spell Progression table from this point on). Casting level continues to rise with experience. Rather than spells, spell-like powers are acquired.