I think the 'reverence' in particular here is a contrast to the blind fervour with which the population follow Big Brother. Under the Party, everything is dictated: who to love, who to hate, to the point where even thought itself is limited. Winston himself says that 'nothing is your own except for the few cubic centimetres inside your skull', but even this is contradicted, both in the clinical classification of size as a potential nod to the erosion of imagination, and the gradual colonisation of minds under the thoughtcrime and doublethink legislation. Yet Winston's 'vague reverence' of the bird is, if nothing else, a glimmer of hope amongst the dystopia: he does not understand why, but he is drawn to the bird on the windowsill, the innocent, natural beauty that permeates event this most barren of worlds. Birdsong as well, particularly in the aftermath of WW1, is a symbol of hope and freedom. Winston is unable to understand why it sings, because he has never known a world where beauty is can exist for beauty's sake alone, and hope is a notion to be entertained, but he is still moved by it. This is both tragic and hopeful, a fitting oxymoron in a book characterised by contradiction in its most impossible sense: the fact that he cannot understand is demonstrative of the horror of the world he lives in, but the fact that he is still moved by it, and feels a 'reverence' that should be reserved for the Party and Big Brother, is more illustrative of hope. William Empson says of Paradise Lost that it is 'the triumph of human courage and emotion over restrictive obedience', and I think this is true here as well. Though Winston's mind is crushed and destroyed in Room 101 and betrays all the things that enabled him to actually feel alive, and begin to understand why he found the birdsong beautiful, there is still the implication that human minds have, and always will be, predisposed to the acknowledgement of beauty and poetry, even if they can be destroyed. And we are back to that idea of hope again, if not for Winston, but for the world as it was, and the world that can be. I suppose in a way, he is right that 'who controls the past controls the future. Who controls the present controls the past.' But that doesn't necessarily have to be a bad thing. It can apply to hope as well.