lovebird vs sweetheart

lovebird

noun
  • One of the members of an openly affectionate couple. 

  • Any small parrot from one of the nine species within the genus Agapornis. Sometimes they are kept as cage birds and are noted for their affection towards each other. 

verb
  • To express romantic affection openly. 

sweetheart

noun
  • A person very much liked or loved by someone, especially when both partners are young. 

  • A person who is always very kind. 

  • A female member of a college or university fraternity. 

How often have the words lovebird and sweetheart occurred in a corpus of books? (source: Google Ngram Viewer )