moorhen vs swamphen

moorhen

noun
  • A female red grouse, Lagopus lagopus scotica. 

  • Any of various medium-sized water birds of the genus Gallinula, of the rail family, that feed in open water margins. 

swamphen

noun
  • Any of several species of bird in the genus Porphyrio of the family Rallidae. 

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