common moorhen vs purple swamphen

common moorhen

noun
  • A member of the species Gallinula chloropus. 

purple swamphen

noun
  • Various species in the Porphyrio genus of the Rallidae family, including Porphyrio porphyrio, Porphyrio madagascariensis, Porphyrio poliocephalus, Porphyrio indicus, Porphyrio pulverulentus, and Porphyrio melanotus. 

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