death march vs mourning

death march

noun
  • Synonym of dead march (“mournful music for a funeral etc.”) 

  • A forced movement of people, on foot, in such circumstances that many die during the journey. 

  • A project that requires a stretch of unsustainable overwork, or whose participants feel it is destined to fail. 

mourning

noun
  • Feeling or expressing sorrow over someone's death. 

  • The traditional clothes worn by those who mourn (in Western societies, typically coloured black). 

  • The act of expressing or feeling sorrow or regret; lamentation. 

  • Drapes or coverings associated with mourning. 

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