No one knows the answer, but the numbers have dramatically decreased not only in this country but worldwide. The usual reasons are climate, farming changes etc. It could even be some sort of disease peculiar to sparrows as yet unidentified. If it was farming or loss of habitat other birds would also be decreasing, but in some cases the opposite is true, some are actually on the increase - only a few species, but still good news.
I think hammerman is right, it's a local thing. If you are blessed with trees and bushes the odds are you will have sparrows. This year in my area has been an excellent year for breeding. The numbers went from 2 to 25 in my garden alone so that is pretty encouraging.
Starlings we never see, but the Thrushes will be making their appearance when the natural food has disappeared and they need to stock up in our gardens on apples or other food. Magpies and Wood Pigeons are the chancers of the bird world and highly adaptable as well as large, so they will be more noticeable than other species.