green woodpecker vs nest egg

green woodpecker

noun
  • A European bird, Picus viridis. 

nest egg

noun
  • A natural or artificial egg placed in a bird's nest, to encourage the bird to lay its own eggs there. 

  • An egg squash. 

  • A saving; a reserve of money. 

How often have the words green woodpecker and nest egg occurred in a corpus of books? (source: Google Ngram Viewer )