alula - What retractable group of feathers is support by a bird's "thumb" bone, two - How many shell membranes does a bird egg have", flyways - What are migration routes that birds follow year after year called?, keratin - Of what substance are the shell membranes of a bird egg made?, preen gland - What organ produces oil that bird spread over their feathers?, keel - What structure on a bird's sternum provides an attachment for the flight muscles?, mobbing - A flock of smaller birds is harassing a larger bird. What is this behavior called?, gizzard - What special section of a bird's stomach grind food to make it more easily absorbed?,

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