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.

• 06.082 Other birds flew to the tree-tops
• 19.011 The wind’s in the tree-top, the wind’s in the heather;

Hyphenated or two words will do for OED.

“ˈtree-top | tree top, n.” OED Online, Oxford University Press, June 2017, http://www.oed.com/view/Entry/205437. Accessed 21 September 2017.

• 02.040 through the tree-trunks not far ahead.
• 02.079 it was thick as a young tree-trunk –
• 06.072 and brushwood round the tree-trunks.
• 07.045 propped on wooden posts made of single tree-trunks.
• 08.003 and scuttling behind tree-trunks.
• 08.109 of the branch nearest the tree-trunk

Hyphenated in its sub-entry in OED.

“tree, n.” OED Online, Oxford University Press, June 2017, http://www.oed.com/view/Entry/205416. Accessed 21 September 2017.

• 07.031 and rows of hives with bell-shaped tops made of straw.

This one’s OED entry is hyphenated exactly thus

“bell, n.1.” OED Online. Oxford University Press, June 2017. Web. 5 September 2017.