The RSPB website also put forth the intriguing notion that House Sparrows have been in decline ever since we began abandoning horse-drawn transport. There was always spilt grain around feeding troughs, even in the urban environment. In recent years, farming efficiency has increased and the harvest is secure inside a grain silo with no spillage worth speaking of, so even the ones in the suburbs are probably dependent on garden feeders for >80% of their diet.
I am as fond of "correlation is not causation" as the next person but I would really like to see the decline in various bird species' populations and the greenfield housing development growth (since 1900) plotted on the same graph, just to see if there is any pattern or no pattern.
For example, if human population is increasing and more and more of us are putting out food for the garden birds, why are the populations of some species declining in spite of that? It's not as if the bird food is harmful to them, is it?