It may be worth while reading the
"Objects of the Society", especially the last paragraph in page 2. Then read the rest.
http://www.rspb.org.uk/Images/charter-and-statutes_tcm9-382216.pdf
For convenience I've posted the relevant section of the charter below:-
3 The Objects shall be:
1) To promote the conservation of biological diversity and the natural environment for the public benefit, in particular but not exclusively by:
a) conserving wild birds and other wildlife, and the environment on which they depend;
b) protecting, restoring and re-creating habitats. And, in furtherance of that primary objective, to raise public understanding and awareness
of, and to provide information on, such matters.
2) To advance education of the public in conservation of the natural environment. In pursuing these Objects the Society shall:
a) undertake activities in Our United Kingdom, in the Channel Islands and the Isle of Man, the UK Overseas Territories and anywhere else in the world; and
b) by so doing seek to persuade the public not only that the beauty of birds and nature enriches the lives of many people but also that nature conservation is fundamental to a healthy environment upon which the survival of the human race depends.
The Society shall take no part in the question of the killing of game birds and legitimate sport of that character
except when such practices have an impact on the Objects.
Shooting interests are quick to claim that the first part of the final sentence debars the RSPB from opposing game shooting but the second part (which I've highlighted) is a significant loophole. The widespread practices of driven grouse shooting put it clearly in breach of the objectives.