barn swallow vs common house martin

barn swallow

noun
  • Hirundo rustica, the most widespread species of swallow in the world; a distinctive passerine bird with blue upperparts, a long, deeply forked tail and curved, pointed wings, is found in Europe, Asia, Africa and the Americas. 

common house martin

noun
  • A migratory bird of the species Delichon urbicum, breeding in Europe, North Africa and temperate Asia. 

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