Clue: Look for bill with extra length — characteristic of a healthy bird? (9)
Answer: SLEEKNESS
Explanation: Definition: characteristic of a healthy bird
I think this is L[ength] in SEEK NESS, with NESS and "bill" both meaning a projection or headland (Chambers gives beak="a pointed projection"). Any other suggestions? *I've just thought of Portland Bill, which probably justifies this equivalence.)