While in the Great Divide, use the Stormfeather's Pride Griffawn Feather that you received for completing Wing Wranglin', hail the griffawn pet and choose "You look very hungry!". This is one of five dialogue choices that are used to train your griffawn into a flying mount, and these dialogues can be chosen once each and in any order every 18 hours.
Note the Cows have two or three Calfs with them that do not give quest credit
Once you get all 12 the quest will auto-complete.
If you have not done all the side-quests yet: in 18 hours you can pick another training lesson for your griffawn!
If this is your final side-quest: Congratulations! Hail your griffawn pet and you will receive another "Quest Complete!" box where you can pick your new griffawn mount, as well as receive some more experience and money. Choose from: Snow Gryphon, Highland Gryphon or Tundra Gryphon. The Stormfeather's Legacy quest series will now be available from Hircus.