4)

If the aerodrome is utilising the ‘long grass policy’ of bird control (as LHR certainly does), around mid-March to May the grass should typically be cut to within 50mm of the surface, either every two or three years, or a rolling programme.

CAP772, http://www.caa.co.uk/docs/33/CAP772.pdf
Chapter 4 Section 2.6.8.1 refers.

PS If anyone wants a laugh, have a look at Chapter 4 Section 3.2.10.2, manual dispersal techniques.