What is the proper usage of a pie bird?
From this site I found that there are two major benefits to using a pie bird



  Prevent the pie from boiling over in
  the oven by allowing the steam created
  when the fruit filling or other
  contents are cooking to escape from
  inside the pie


  Support the pastry crust in the center
  of the pie, so that it did not sag in
  the middle



Based on the site it seems that they can be put in any type of pie.