http://www.geocities.com/traditions_uk/august12.html Site no longer active so this Wikipedia page is a good alternative:

https://en.wikipedia.org/wiki/Glorious_Twelfth

From the page:
“The Red Grouse (Lagopus lagopus) is a medium-sized game bird found only in the British Isles, being a subspecies of the Willow Grouse of Europe, Asia and North America.

It breeds on the heather moors of northern Britain and its numbers are carefully maintained, despite the annual slaughter, by managed breeding and the controlled burning of the heather to provide the variety of stages of growth in the heather which suits the birds best – young shoots for food, mature growth for shelter, and clearings for the chicks and young birds to sun themselves in.

The grouse are allowed to breed unmolested and the season’s chicks will be fully mature by the beginning of August, when the close season comes to an end on the 12th of the month.”