gallinule vs swamphen

gallinule

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

swamphen

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

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