The occupation of searching for birds’ nests.  It is hyphenated thus in OED.

[06.076] ‘Go away! little boys!’ shouted Gandalf in answer.  ‘It isn’t bird-nesting time.

• 06.076 It isn’t bird-nesting time.

“ˈbird’s-ˌnesting | bird-nesting, n.” OED Online. Oxford University Press, June 2017. Web. 5 September 2017.