It's House Sparrow Day, but where have all the houses gone?

Much has been made of the "extinction" of house sparrows. Rural countrysides where farmers practice agriculture traditional crop cycles still have sizeable populations of the birds. At dusk, small trees tremble with fluttering boarders settling in to roost. Their frenzied chirruping, like the jingle of millions of coins in thousands of fists, mounts to a crescendo just before daylight fades away.

What's the real reason for the decline of house sparrows? Wait, is it really a decline? Or is just that the populations have moved elsewhere? Is there a real reason for alarm, or are we inclined to jump to romantically motivated conclusions regarding the absence of these birds from our cities? Should we point fingers at electromagnetic radiation from microwave towers? Or unleaded petrol, which contains the anti-knocking agent Methyl Tertiary Butyl Ether (which is known to be fatal to insects with which adult sparrows feed their young)? Or the decline of traditional architecture that offered nest-friendly eaves? Does it have to do with the pesticide content in our foodgrains? Or the profusion of crows? 

Theories abound but, as of now, no one knows for sure. Meanwhile, let's persist with those nest boxes.