do only the lower tier sororities do spring rush?

Sororities are allowed to participate in Spring Rush if they are below "chapter total." Chapter total is a number set by Greek Life that for the most part represents the average size sorority at LSU. This means that half of the sororities are above this number and half are below this number. Sororities can be below this number for many reasons. Some sororities have more girls go to nursing school, some sororities allow girls to go "alum" early, and some sororities have more girls drop than others for any given reason. No sorority is required to participate in spring recruitment though! Most sororities get enough girls in the fall (I can only think of one who didn't make quota my year), so it's not usually about that. Hope this helps!

Also, some national offices require their to participate in the extra recruitment if they are below total, others make it optional.