The Lightning Returns: Final Fantasy XIII side quest named Saving An Angel starts after you save the white Chocobo and return to Canopus Farms, but only if you talk to Dr. Gysahl.
The side quest is part of Healing An Angel main-quest, both of them being completed at the same time.
To begin Saving An Angel, talk to Dr. Gysahl and take the Gysahl Greens from him. Dr. Gysahl wants you to feed the injured bird using the Greens he gave you.
In Saving An Angel you must find more food for the injured Chocobo.
While there are many ways to complete Saving An Angel, the following walkthrough provides the fastest, allowing you to save time and have your Chocobo back on his feet as soon as possible, so you can focus on other side-quests.
Description: “Thanks to the desperate efforts of Dr. Gysahl and his staff, the Angel of Valhalla is out of immediate danger. But now, he needs food to regain his strength. It is time to see if Lightning really is the chosen one. She must take the Gysahl Green from the doctor and try feeding it to the proud Chocobo.”
Location: The Wildlands
Requirements: Complete main quest 3-1: The Angel of Valhalla. Start main quest 3-2: Healing An Angel
Rewards: 1200 Gil, Maximum HP + 80, Strength + 8, Magic + 8, White Chocobo Chick Adornment
After you give the seeds to the Chocobo near Dr. Gysahl, you will be informed that it’s not enough; therefore, you need more food and you have to get it as quickly as you can.
In the Wildlands, without the Chocobo, you will waste a lot of time, crossing the large areas. Open your map, and in Canopus Farms, go to the Inn. Talk to the Innkeeper, and you will receive Fuzzy Sheep Milk.
Next, turn around and head west, to reach the farm’s entrance. Stop when you get close to the entrance and talk to Sarala, the girl on the right side of the road. She has red hair and she stands close to a haystack and two carts. Sarala will give you more Gysahl Greens.
Several steps away from Sarala, is a man riding a yellow Chocobo. It is the Hunter Chief, and you have to talk to him to get Chocobull.
Now, open your map and go to Aryas Village. Aryas Village is south from Canopus Farms, and also south from Aryas Climb, between the two paths leading outside the Wildlands.
Note: Activate the Chronostasis ability while traveling to and back from Aryas Village.
When you get to Aryas Village you have to find the Dr. Sheep and finish the Fuzzy Search side-quest for which you will receive Fuzzy Sheep Milk. With all items in your inventory, go back Canopus Farms and feed the Chocobo, to finish both Saving an Angel and Healing an Angel quests. Next, you should complete the Chocobo Cheer side-quest.