Doing shrugs in the push up position (also called push ups +) will more directly work the serratus.
Stu makes a good point - doing shrugs on a parallel bar doesn't put the serratus muscle in a strong position to do work.
If you look at the serratus exercises listed on this site
, the common element is moving the shoulders forward (albeit on an upward incline), not directly upward. And overhead presses
work the serratus, as well.
The biggest problem with the serratus is that you have to be pretty lean for them to be impressive looking