The 1000-year bird song

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Across
  1. 3. the intention, aim or function of sth; the thing that sth is supposed to achieve
  2. 4. a problem, rule or situation that prevents sb from doing sth, or that makes sth impossible
  3. 7. done, chosen, etc. without sb deciding in advance what is going to happen, or without any regular pattern
  4. 9. to keep sb/sth alive, or safe from harm or danger
  5. 11. the act of joining or mixing together two or more things to form a single unit
  6. 13. any of the units into which a word is divided, containing a vowel sound and usually one or more consonants
  7. 14. one of the things from which sth is made, especially one of the foods that are used together to make a particular dis
  8. 15. to copy the way sb speaks, moves, behaves, etc., especially in order to make other people laugh
  9. 16. to separate sb/sth physically or socially from other people or things
  10. 17. not very often
  11. 18. to make a copy of a picture, piece of text, etc
  12. 19. to recognize sb/sth and be able to say who or what they are
  13. 20. firmly fixed; not likely to move, change or fail
Down
  1. 1. In music, a note is the sound of a particular pitch, or a written symbol representing this sound.
  2. 2. a living thing, especially one that is extremely small
  3. 3. that can develop into sth or be developed in the future
  4. 5. a group into which animals, plants, etc. that are able to breed with each other and produce healthy young are divided, smaller than a genus and identified by a Latin name
  5. 6. not long ago
  6. 8. a series of musical notes that are sung or played in a particular order to form a piece of music
  7. 10. long life; the fact of lasting a long time
  8. 12. correct and true in every detail
  9. 17. to make sth known to sb