gallinule vs purple swamphen

gallinule

noun
  • A bird of one of several species in the genera Porphyrio and Gallinula of the family Rallidae. 

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 gallinule and purple swamphen occurred in a corpus of books? (source: Google Ngram Viewer )