Sparrows have never been great singers - when you whistle like a bird I think it's most likely to be a blackbird you're instinctively copying.

I think sparrows may be on the way back - several times this year I've seen a bunch of them. A long way from being common, the way they were, but it's a start. Noone has come up with an agreed reason for why they almost vanished.
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Perhaps whistling in the street has fallen victim to a sort of pincer movement between those who think life should be business like, and those who think that life should be approached with a cynical sneer. Whistling your way through life doesn't fit in with either attitude. Whistling as a kind of resistance movement...