The letter asks politicians to take various measures to conserve wildlife, and makes clear that the main reason for doing this is Our Children:
I’m writing this now to make sure our children have a chance of growing up in a world worth living in. ...
If we act now, our children may yet be able to share their world with sparrows and polar bears, eagles and tigers. There’s still a chance that they’ll inherit a world where the engines of life – the air, seas, rivers and forests – are healthy. Where bluebell woods and rainforests won’t be lost forever. ...
I’m signing this letter to show that I care deeply about nature and the world we are creating for our children. In years to come I hope they’ll be able to see that their world is a richer one because of the action we took today.
Next to the space for signatures is a blank box headed, "We'd love to know your reasons for signing". Presumably the RSPB wants to gather some quotations from supporters that it can use in its publicity, as it has in the past. One of their adverts a few years ago featured a member's claim that "I didn't join for the birds, I joined for my kids" (am I a bad person because that makes me nauseous?).
I wrote: "I'm not signing because of Our Children -- I have none -- but because wildlife has an inherent value and right to exist, which does not depend on its usefulness or interest to humans, either present or future".
But I don't know if they can fit that on a poster.