house martin vs northern house martin

house martin

noun

  • The common house martin (Delichon urbicum). 

  • A migratory passerine bird of the swallow family, genus Delichon 

northern house martin

noun

  • The common house martin (Delichon urbicum). 

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