bilberry vs cowberry

bilberry

noun
  • The shrub of the above-mentioned plant. 

  • Vaccinium myrtillus, the wild European blueberry of the cowberry family. 

cowberry

noun
  • The berry of this shrub. 

  • A shrub native to the cool temperate areas of the Northern Hemisphere (Vaccinium vitis-idaea). 

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