The incantation for the Bird Conjuring Charm is Avis. This spell is used to make a number of small, twittering birds fly out of the tip of a wand.
From the Story
Discovered in Book 4, Chapter 18, The Weighing of the Wands, Moment 2 Rita Skeeter
"Mr Ollivander uses this spell to test the functionality of Viktor Krum's wand during the Wand Weighing ceremony."