No mow May is good. For birds, no mow patches all year is better.

 No mow May provides a feast for pollinators, tackles pollution, reduces urban heat extremes, and locks away atmospheric carbon below ground. 

Reference: https://www.plantlife.org.uk/campaigns/nomowmay/?gad_source=1

In this Cuckfield, West Sussex garden patches of Common Knapweed have been left to set seed and are unmown up to now.  The result has been glorious.  A flock of five Goldfinches and a Blue tit fluttered all over dead stems feasting on the seed heads earlier this week.