Possible Clues:

• A very rare bird, of course
• Double eagle!
• A score of 3-under par for hole of golf
• The bird of The Ancient Mariner
• As Borstal is adapted for a big bird
• Getting around round around a round five
• Coleridge's bird
• Coleridge's marine bird.
• Bird outperforming an eagle
• Birdie surpassed by Coleridge's handicap ...