cowberry vs whortleberry

cowberry

noun
  • The berry of this shrub. 

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

whortleberry

noun
  • A berry of one of these shrubs. 

  • Vaccinium vitis-idaea (lingonberry or red whortleberry); 

  • Vaccinium myrtillus (bilberry or blue whortleberry); 

  • Vaccinium uliginosum (bog whortleberry or bilberry). 

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